Change user mentions in a post to their URL

[joshsmith]

We'll want to borrow heavily from this: https://github.com/twitter/twitter-text/blob/master/js/twitter-text.js#L643

Closed by #93.

[joshsmith]

Another approach, though, while I'm thinking of it, is to sort backwards by index. That is, you can do string replacement backwards because the first indices will have unchanged character placement.

Here's an example of this in action: http://stackoverflow.com/a/30388998/373496