append new elements for each element in selection

fixes #438
fixed context of `this` in .html
Merge pull request #457 from cheeriojs/html_fix

Append new elements for each element in selection
Use "loaded" instance of Cheerio in unit tests

This usage allows tests to be slightly more concise. In addition, it
better reflects the real-world usage of Cheerio, promoting the unit
tests' role as active usage examples.
Merge pull request #504 from jugglinmike/test-loaded-cheerio

Use "loaded" instance of Cheerio in unit tests
Implement `Cheerio#add`