Make kerbdown regex less greedy

If you have 2 or more embedded links to youtube/imgur parser produces incorrect output.

If first embedded object is youtube video, it produces 1 embedded youtube video with almost all between first `[[` and last `]]` in markdowned doc. If first link is imgur image, it produces correct embedded objects but drops all before last `]]`.

This commit fixes such behavior.
Merge pull request #78 from grossws/patch-1

Make kerbdown regex less greedy