Add support for setting the sw, sts, and comment string in an ftplugin.

[ssteinbach]

The idea is to borrow the lua ftplugin from:
https://github.com/xolox/vim-lua-ftplugin/blob/master/ftplugin/lua.vim

And tack on the pico8 special sauce.

[ssteinbach]

Another question - Is there a way to ignore the first two lines of the file/everything after the text block? Or maybe show colors for the stuff in the sprite sheet?