This Python script uploads cards from a given text file into Anki. More in comments ⬇️

anki.py

import re
import sys
import getopt
import json
import urllib.request


def request(action, **params):
    try:
        requestData = json.dumps(
            {"action": action, "params": params, "version": 6}).encode("utf-8")

        response = json.load(urllib.request.urlopen(
            urllib.request.Request('http://localhost:8765', requestData)))

        return response
    except:
        print("Can't connect. Make sure that you've AnkiConnect installed: https://foosoft.net/projects/anki-connect/")


def invoke(action, **params):
    response = request(action, **params)

    if len(response) != 2:
        raise Exception('response has an unexpected number of fields')
    if 'error' not in response:
        raise Exception('response is missing required error field')
    if 'result' not in response:
        raise Exception('response is missing required result field')
    if response['error'] is not None:
        raise Exception(response['error'])
    return response['result']


def list_deck_names():
    return invoke("deckNames")


def list_model_names():
    return invoke("modelNames")


def add_notes(deck, model, notes):
    invoke("addNotes", notes=list(map(lambda note: {
        "deckName": deck,
        "modelName": model,
        "fields": {
            "Front": note[0],
            "Back": note[1]
        }
    }, notes)))


def print_help():
    print("""
This script uploads cards from a given text file into Anki.
AnkiConnect extension needs to be installed: https://foosoft.net/projects/anki-connect/

Every line in an input file should match this format:
- back - front

Fronts cannot be duplicated.

Available options:
    -h, --help                 Prints this message.
    -f, --file=FILENAME        Sets the path to the input file. The default is "Words.md".
    -d, --deck=DECK            Sets output deck name to which cards are append. The default is "English".
    -m, --model                Sets model (type) of cards. The default is "Basic".
    -r, --regex=PATTERN        Changes regular expression used to read front and back from a line. The default is "-\s+(.*)\s+-\s+(.*)".

Example usage:
    python anki.py --file="notes.txt" --deck="Spanish" --model="Basic (and reversed card)"
""")


def main(argv):
    regex = r"-\s+(.*)\s+-\s+(.*)"
    file = "Words.md"
    deck = "English"
    model = "Basic"

    opts, args = getopt.getopt(
        argv, "hf:r:d:m:", ["help", "file=", "regex=", "deck=", "model="])
    for opt, arg in opts:
        if opt in ("-h", "--help"):
            print_help()
            sys.exit()
        elif opt in ("-f", "--file"):
            file = arg
        elif opt in ("-r", "--regex"):
            regex = arg
        elif opt in ("-d", "--deck"):
            deck = arg
        elif opt in ("-m", "--model"):
            model = arg

    file = open(file, "r")
    translations = {}

    for line in file.readlines():
        line = line.strip()
        if len(line) == 0:
            continue

        search = re.search(regex, line)
        if search is None:
            print("Suspended, unparsable line:", line)
            print(
                "Every line in an input file should match this format: \"- back - front\"")
            return

        (back, front) = search.groups()
        if front in translations:
            print(f"Suspended, duplicated card fronts:")
            print(f"    1. {front} | {translations[front]}")
            print(f"    2. {front} | {back}")
            print("Please remove duplicated fronts and run the script again.")
            sys.exit()

        translations[front] = back

    print(f"Successfully loaded {len(translations)} translations!")

    if deck not in list_deck_names():
        print(
            f"You don't have a deck named {deck}.")
        print("You can use another deck with --deck=\"your's deck name\" param.")
        return

    if model not in list_model_names():
        print(
            f"You don't have a model named {model}.")
        print("You can use another model with --model=\"your's model name\" param.")
        return

    add_notes(deck, model, translations.items())

    print(f"Successfully added {len(translations)} notes to Anki!")


if __name__ == "__main__":
    main(sys.argv[1:])

This script uploads cards from a given text file into Anki.
AnkiConnect extension needs to be installed: https://foosoft.net/projects/anki-connect/

Every line in an input file should match this format:
- back - front

Fronts cannot be duplicated.

Available options:
    -h, --help                 Prints this message.
    -f, --file=FILENAME        Sets the path to the input file. The default is "Words.md".
    -d, --deck=DECK            Sets output deck name to which cards are append. The default is "English".
    -m, --model                Sets model (type) of cards. The default is "Basic".
    -r, --regex=PATTERN        Changes regular expression used to read front and back from a line. The default is "-\s+(.*)\s+-\s+(.*)".

Example usage:
    python anki.py --file="notes.txt" --deck="Spanish" --model="Basic (and reversed card)"

Update!
I've wrote a CLI tool with the same functionality in Elixir: https://github.com/arturz/anki_connect#command-line-interface-cli (scroll down to CLI section)
Updates and new features will be added to this Elixir package.