(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 to 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.

If you have any ideas to fix the bugs without having the design changed, let me know. I would like to work on it. Let's discuss it further. Thanks!