Initial pass at implementing max_active_tasks feature

@coddingtonbear Want to take a look when you have a moment? This (`task +ACTIVE status:pending count rc.verbose:off`) is returning the task ID for the task I'm starting when passed through `taskw`; if I run the command in the terminal it returns what you'd expect.
Hrm, yeah, it doesn't look like the `filter_tasks` command was really set up with `tags` in mind.  Also -- if you're hopping into the (implicitly private) `_execute` anyway, you might want to consider just shelling-out to `task` using `subprocess.Popen`; you can then use the +ACTIVE tag however you want.   If you do want to stick with `_execute`, though, keep in mind that it expects the param to be an array of args (per unix tradition); in your case it'd be something like `['task', '+ACTIVE', 'status:pending', 'count', 'rc.verbose:off']`.