`Executor.filter` is to `filter` what `Executor.map` is to `map`.

See Python-ideas thread:

https://groups.google.com/forum/#!topic/python-ideas/EBOC5YCWPyo

Patch attached. I don't know how to run the Python test suite (I'm guessing it involves building Python and I don't know how to do that.) Please tell me whether the tests I wrote pass.

I'm guessing that other than that, the main thing missing in my patch is documentation. If there's agreement that this feature is desirable and the implementation is good, I'll be happy to write the docs. Let me know.