This commit removes the 'main' function present in main.rs, renames it to exa.rs, and puts the 'main' function in its own binary. This, I think, makes it more clear how the program works and where the main entry point is.
Librarification also means that we can start testing as a whole. Two tests have been added that test everything, passing in raw command-line arguments then comparing against the binary coloured text that gets produced.
Casualties include having to specifically mark some code blocks in documentation as 'tests', as rustdoc kept on trying to execute my ANSI art.
