Version 0.18.0

Changed

• Raise ValueError if quote_mode isn't one of always, auto or never in set_key (#330 by @bbc2).
• When writing a value to a .env file with set_key or dotenv set <key> <value> (#330 by @bbc2):
  • Use single quotes instead of double quotes.
  • Don't strip surrounding quotes.
  • In auto mode, don't add quotes if the value is only made of alphanumeric characters (as determined by string.isalnum).