Allow specifying arbitrary include paths

guard-minitest already allows specifying `test_folders`, but those are also used for filtering test files, so if one wanted to add `test` to include paths, but only run tests from under `test/unit`, it would not be possible with the current options.

This pull request adds a secondary `:include` option, akin to the one that [guard-test][1] uses.

[1]:https://github.com/guard/guard-test