add .data() function. cleaned up PR #221 from @rafaelrinaldi, added tests, updated readme

Since the .data() function was implemented by @rafaelrinaldi PR#221, but on an older version of cheerio, it didn't make it back into the original cheerio.

I couldn't wait for a solution on this, therefore I fixed this thing now.

I cleaned it up (with the up to date version), added tests and updated the readme.