Fix infinite loop and link-in-link-in-image parsing (#37)

There was a bug in the logic which ensures that links don't contain links.  See https://github.com/jgm/CommonMark/issues/252 for additional details.
Remove unused variable
Merge pull request #41 from thephpleague/fix-infinite-loop

Fix infinite loop and link-in-link-in-image parsing (#37)