Use the HTTP regex to match a URL before opening it.

I noticed that using markdown formatting, pressing enter on a URL won't work.

Example:
``` The URL of [Google](http://www.google.de)```
results in the command
```gnome-open '[Google](http://www.google.de)'```

I  fixed that by matching the `http_link_pattern` on the word expansion to get rid of the non-URL stuff.