Support configuring file extensions for markup formats?

[swapneils]

Can we have an option to extend the set of default file extensions to include files which are effectively text but do not use the .txt extension?

I see #26 regarding Markdown files, but this is a more general problem; .tex (LaTeX), .bib (for citations), .org (for the Org markup format), and a variety of code extensions should be as visible to the application as any other text file, but are ignored due to their file extension.

Given it's near-impossible to make a default list of all the world's different text-file extensions, users need to be able to configure file-extension aliases ourselves, which means we need a UI feature allowing such aliases in the application settings.