Adding logic for charm tags.

Adding functionality to tag charms. This is a pre-cursor for charm annotations :)
This doesn't need to be exported.
We have a lot of charm URL related parsing and validation functionality in juju/charm. As it stands, the valid charm name and series regexp is duplicated here and in juju/charm. As a follow up, consider what might be done to juju/charm to remove the duplication. That repo already imports juju/names so maybe there's some common functionality that can be moved to juju/names