Python hello world example for the exec system

exec_helloworld.py

#!/usr/bin/env python
# Simple hello world exec script showing basic usage

import sys
import os

username = os.environ.get('MCEXEC_PLAYERNAME')

if username == None:
  # yellow is a good colour to use to warn players about problems
  print("§eError getting username from exec!")
  exit(1)

# green is a friendly colour to use for output from a command that succeeded
print("§aHello, §f%s§a!" % username)