Add command to filter by test `@tag :tag` attributes.

[mikowitz]

Adds the t command to the interactive runner.

Usage

t tag1 tag2

will append --only tag1 --only tag2 to the end of the test command being run, ensuring that only tests with @tag :tag1 or @tag :tag2 are run.

Like the p command, the list of tags can be cleared by passing t alone to the interactive runner.

[randycoulman]

@mikowitz Thanks for the contribution! I'm quite busy for the next week or so, but I will review this as soon as I get some free time.

[mikowitz]

@randycoulman sounds good! Thanks for building the bulk of this tool, and for making it so easy to add functionality.

[randycoulman]

I'm going to close this PR as stale. I am planning to work on tag filtering features in the coming weeks, and I think it'll look a little bit different than what's here. In addition, the recent changes for "config-less operation" mean there's some work to do to rebase this.

Thanks again for the contribution!