Add method for purging Author Pages

Resolves WebSharks/Quick-Cache#80

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@JasWSInc Could you please review this for me when you get a chance and provide any feedback?

The main thing here is that in the event that the author for a post is changed before publishing, we need a way of handling both the current author and the previous author. In that scenario, both author pages should be purged (but only when the post status makes sense for purging). Please see my comments in the source code.

This could probably use quite a bit of performance enhancement, since it's attached to the `post_updated` hook. (Related to Reduce Auto-Purge Overhead #130.)