Allow customizing builtin extension colours

[ariasuni]

exa is currently coloring some files based on filenames and filetypes in a non-trivial way that makes hard or impossible to replicate using only EXA_COLORS.

Contrarily to what the man says (“exa provides its own built-in set of file extension mappings […]”), exa uses a few algorithms, like checking for files starting with readme case-insensitively, or checking if some file is compiled if it has a source (e.g. mark CSS as compiled if there is a SASS source).

Even if someone doesn’t want those advanced features and could live with just a basic coloring based on extension, it’s really handy that exa already knows about the most popular and it’s really hard to manage an EXA_COLOR for frequent filetypes because of the basic syntax.

It would be great to be able to use EXA_COLORS to set the color of whole file types, instead of needing to set a color for every file extension and still missing the possibilities and extensive list of file extensions maintained in exa.

It would fix #338 and #363 (and close #370) in my opinion.

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I tried to see how it can be done, based on what has be done in #607, but I couldn’t see how to make it work, so I gave up (at least for now).