Updating parseHtml to handle an empty url, updating cheerio version and README

Really like this library, appreciate your hard work. I was trying to parse html bits ad-hoc and ran into two issues.

1. Had to have a url in the parseHtml method
2. Signature of parseHtml was incorrect in the README

I saw you also had a dev branch.  I can make changes there if need be.

Also, I updated cheerios to the latest and greatest.