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

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.