Add support for Google Docs footnotes

It's a great add-on and I personally love being able to decouple from the editor and write from a more natural one, but It appears that footnotes are not currently supported. That would be a wonderful feature for those of us who use footnotes.

The issue names no files, tests, or entry points. Start by locating the add-on's Google Docs content handling and determine how footnotes should be represented when publishing to WordPress; done means footnotes are preserved with agreed formatting and covered by tests.

Written by the indexing model from the issue text.