Support for binary Extras, e.g. images, PDFs

[hparra]

Allows two related features:

• Copying of binary Extras for inclusions in markdown files (e.g. images), while retaining its relative path for viewing in both Github et. al. and generated doc.
• Listing and linking of binary Extras, allowing display in main frame if browser supports format, e.g. images, PDFs.

Pre-existing testing was limited, but I was able to add one simple test. Changes have been smoked with real project.

Entities.Extra determines if file is binary and holds its buffer, while Theme.Templater#render() writes the buffer (instead of rendering a template) if it exists.

Theme.Theme#pathFor() helper no longer affixes ‘.html’ if buffer exists.

While it may have been more ideal to create a new type of Entity, doing so would have resulted in significant duplicated code and additions to the CLI interface.

[inossidabile]

I know it took a while. But could we rebase this?

[inossidabile]

Closing for now.