Fix incorrectly assigning selected class to index

I've simply added ^ and $ to the regex fed to the highlights_on option so now
it only highlights exact matches and not partial matches.

[fix #3]
Merge pull request #6 from relu/selected-item-class-fix

Match entire path for highlights_on. I'm not positive this will always be correct (because of file extensions) but it's a better starting point that the reckless match in the original code.