Several ES6 additions are only available in strict mode. Example:
> SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
Strict mode was also enabled in a few of our files already, and it is a good thing to have anyway.
Fixes the /part command closing the wrong window. The current implementation simply passes all arguments to slate, which ended up parting every arguments.
This changes the command to `/part message`, and always parts the current window. This will be fixed further once irc-framework is merged.
