Closes #546: Provides a hook to allow a plugin to optionally expand any links in the body text. 

Note: Since this is likely to be called several times, it should be done fairly efficiently. Probably cached.
Passing the object in question, since this provides a way of caching short urls until #371 is merged
Merge pull request #547 from mapkyca/expanding-short-urls

Adds a hook to expand on short URLs in body text