feat: add support for configuring the MIX_ENV

[stocks29]

Fixes #138

Allow the MIX_ENV tests run under to be specified in config

[jarlah]

isn't this same as

# mix.exs
def project do
  [
    ...
    preferred_cli_env: [
      "test.watch": :test
    ],
    ...
  ]
end

[stocks29]

It's been a while since I looked at this but from what I recall this line was the problem:

https://github.com/lpil/mix-test.watch/blob/master/lib/mix_test_watch/port_runner/port_runner.ex#L50

|> (fn command -> "MIX_ENV=test #{command}" end).()

It indiscriminately runs tests with MIX_ENV=test regardless of other configuration. While the mix test.watch CLI command will respect the preferred_cli_env, the subprocess which mix_test_watch kicks off ignores that configuration and always uses the test env.

Perhaps I'm missing something though? @jarlah @lpil are you able to run tests via mix_test_watch under custom environments with preferred_cli_env set as described above?