Enable config environment variables over .zunit.yml

[danielbayley]

It would be nice to be able to simply set the .zunit.yml config options with environment variables, and avoid having to manage another file when it only contains a few lines… options like ZUNIT_DIR_TESTS and ZUNIT_DIR_ SUPPORT for example.

[molovo]

I'd rather not add environment variables, since everything else can be overridden via CLI options. I will look into adding --test-dir and --support-dir options though, since they are currently the only options that can't be overridden. Should be simple enough to do.