Elixir: Allow configuration of test runner color output

[x0ul]

My request is a way to allow users the option to configure colors in test output.

In lsp-elixir.el --no-color is a hard-coded flag passed to mix test. Emacs users using ansi-color can take advantage of colors in their compilation window, which are nice in Elixir because there's some colored diff output if an assertion fails.

I've removed the --no-color flag from my local version of lsp-elixir.el and it's working as expected with ansi-color enabled. Next steps would be a way to make color/no-color a configuration option.

I'm an elisp newb, but I may attempt it if I have time.