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.
I think you might've accidentally indented this one level too many; this condition can't ever be true if the one on line 24 is true.