Add --include switch to limit execution to specific contexts/vows

This is an attempt to close #22 by adding a new cli switch. The new switch (--include) works similarly to the exclude switch except the search for the regex happens against the "full name" of the context/vow (e.g. TheGoodThings.ABanana.WhenPeeled.returns_a_peeled_banana).

Example usage:
pyvows --include returns_a_peeled_banana
pyvows --include WhenPeeled.*

Limitations I know about:
* The --include and --exclude switches are mutually exclusive for simplicity
* There is currently no way to specify only running a specific instance of a generated topic
* Because Context.ignore can dynamically change what is executed, it is theoretically possible for a user to --include something that eventually gets ignored anyway