(fixes #104) Repeat mentions with allowRepeat option

A quick fix that enables duplicate mentions.

I added an option called "allowRepeat" which, when it's set true, allows users to mention the same person multiple times. Without changing the original design, I personally think this is the best solution. it seems inevitable, by design, that there will be certain bugs such as the same person getting highlighted without triggerChar. I highly recommend that we work on a new design that has a legit parser.

I would like to discuss it further. Thanks!