Support applying and watching directories.
Support applying and watching directories.

Pass a directory to --css and we will watch and apply changes within that
directory. (We will only look at files with the .css extension).

This change introduces a new restriction that single watched files also need to
have the css extension.

Tested: Windows Discord Canary, watching single file, watching directory,
adding, removing, editing, renaming within that directory, running
applyAndWatchCSS in console. Everything seems to work.

leovoel/BeautifulDiscord