Add compatibility with gemoji v2

This adds compatibility with upcoming gemoji v2 release that has an API completely incompatible with the previous one. I opted for supporting both APIs here because it was easy to do feature detection and so that html-pipeline users can update this library without having to necessarily upgrade gemoji in their apps.

The Gemfile and tests are still configured to use gemoji v1.x for the time being since test output (image names, to be precise) can be different when running against gemoji v2.

/cc @aroben @josh
Also, one can now override the method in a subclass :wink: