feature: configurable filetypes

[BerkeleyTrue]

I have some code that runs in markdowns under certain conditions, which require changing the filetype of certain markdown files to 'markdown.corpus'. It doesn't change anything about the syntax that should mess with Glow rendering.

I would like to extend allowable filetypes, which is currently hardcoded.

It would also be acceptable to have this library look for the root filetype instead of the whole ft match, like how "javascript.jsx" still counts as a javascript file for most plugins.

[AlejandroSuero]

@BerkeleyTrue if you want to use my branch in the mean while and tell me if that way it covers your issue?