Avoid counting lines of code for tests

[franfrandev]

Hi! Thanks for building this :)
Just needed to count lines of code of a Rust codebase while excluding tests for scoping purposes, but it didn't appeared that any tool offered this currently. Is that something that you would like to support in tokei? It would probably require a lot of code in order to make it work for every language, since each of them has its own quirks regarding tests structure. But this branch adds this for the Rust programming language: fa44e51...iFrostizz:tokei:filter_out_rust_tests
Alternatively, maybe that an option --exclude-tests could be added while adding a notice that shows all of the languages currently supported while the feature is being built.