ExUnit: Allow including/excluding subtags

[Айрат Бадыков]

For example,

defmodule Test do

  @moduletag :outer

  

  test "test1" do

  end

  

  @tag :inner

  test "test2" do

  end

end

I want to include test1 and skip test2.  But running 'mix test --include outer --exclude inner' includes both tests.

I think, it's possible to consider precedence of --include/--exclude options or simply just run intercrossing tests.