Format mardown link properly

[gpollo]

Right now, a username is detected if the first character of a work in the tweet starts by the "at" (@) symbol. It stops the reading of the username when the first non alphanumeric/underscore (_) character is found.

Which means that This is a @username. is going to be formatted as This is a @[username](https://twitter.com/username)., with the dot at the end.

The current implementation doesn't support a user reference that doesn't start with an "at". For exemple Weird dot .@username is going to be Weird dot .@username.

We should make sure that #13 doesn't have the same problem.