document annotator plugin

The document plugin extracts metadata from HTML pages (Dublin Core, Facebook OpenGraph Protocol, Google Scholar, link relations) to be sent along as part of the JSON payload for the annotation. 

It is part of some [work](https://github.com/hypothesis/h/wiki/cross-format-annotations) being done at Hypothesis to enable the annotation of documents independent of their format (e.g. html, pdf) and location (URL, DOI, etc). But, as a plugin, with unit tests, it is a discrete bit of functionality that I think can be considered for inclusion in Annotator.

Feedback welcome!