Default (Windows).sublime-keymap

[
    { "keys": ["ctrl+super+f"], "command": "make_footnote"}
]

MyFootnote.py

import sublime, sublime_plugin

# Encode and Decode functions stolen from:
# http://stackoverflow.com/questions/1119722/base-62-conversion-in-python

import string
BASE_LIST = string.digits + string.ascii_letters
BASE_DICT = dict((c, i) for i, c in enumerate(BASE_LIST))

def base_decode(string, reverse_base=BASE_DICT):
    length = len(reverse_base)
    ret = 0
    for i, c in enumerate(string[::-1]):
        ret += (length ** i) * reverse_base[c]

    return ret

def base_encode(integer, base=BASE_LIST):
    length = len(base)
    ret = ''

    while integer != 0:
        ret = base[integer % length] + ret
        integer /= length
        integer = int(integer)

    return ret




class MakeFootnoteCommand(sublime_plugin.TextCommand):
    def run(self, edit):
        # Get the last used footnote number from a permanent, global
        # setting.
        settings = sublime.load_settings('MyFootnote.sublime-settings')
        footnoteLast = settings.get('footnote_last')

        # Decode this number into an integer, iterate and re-encode
        # it
        integerFootnote = base_decode(footnoteLast)
        integerFootnote += 1
        footnoteNew = base_encode(integerFootnote)

        # Update the settings file
        settings.set('footnote_last', footnoteNew)
        sublime.save_settings('MyFootnote.sublime-settings')

        # Insert a markdown footnote with the new number at the
        # current first cursor position.
        pos = self.view.sel()[0].begin()
        self.view.insert(edit, pos, "[^"+ footnoteNew + "]")
        par = self.view.find('\n\n+[\w>#]', pos)
        if par.begin() > 0:
            self.view.insert(edit, par.begin(), "\n\n  [^"+ footnoteNew + "]: ")
            self.view.sel().clear()
            pos = self.view.find(': ', par.begin())
            self.view.sel().add(pos.end())

MyFootnote.sublime-settings

{
	"footnote_last": "2W"
}