support filtering on tags

We would like to be able to do this in order to take advantage of our work on bug 974622. Could someone give us direction on how you would like to see this happen, so we can contribute a branch?

test_results.TestResultFilter looks like it needs to keep track of tags. Would you like to then send them along to the existing predicate function, or would you like a new tags predicate function? I would choose the latter without direction. How would you like the filtering to work on the commandline? Without direction, I would probably guess that --with-tags and --without-tags, with regex support, would best match the existing interface (the --with and --without options).

Here's something that I had in progress. Sorry for not publishing it sooner. I had shared it on #subunit and not bothered beyond that. Note that it's filtering of tests by tags.