Allow feed handlers in theme directories (with patch)

To make maintenance easier, I wanted my feed handler to live in my theme's directory rather than in the Aggregation module's directory. (This is a custom feed format and wouldn't be of use to anyone else).

The attached patch checks the theme directory for the appropriate handler followed by the aggregation modules directory. I suppose to truly optimize this, I should've done it the other way around... Anyhow, see attached.