irccloud upload to imgur

Add option to in file upload menu to upload to imgur instead of irccloud's cdn. This could be used to work around IRCCloud's free account's 100MB upload limit.

This currently does not support uploading to an account. All uploads are anonymous. Imgur's api rate limit is shared by all people with the same client ID. If you are having problems with the provided client ID, you can register one yourself and replace the const CLIENT_ID at the top of the script.