Don't force /usr/bin/tidy, use the PATH, Luke

First of all, this is amazing. I was looking to implement valid HTML5 checking in my test suite and you solved that for me, so bravo.

However, I was dismayed to find out that the HTML5 version of tidy that I'd installed via homebrew was being ignored in favor of the ancient version installed by MacOS in /usr/bin because hmtl_validation wasn't respecting my path. I have my path set up this way for a reason, as I imagine most good devs do :) Please respect the path. That is all.