Detect if inside frame, style accordingly

[nawroth]

Replace the navbar with something minimal, like an open-in-new-window link only.
(We might want the button to say it's a DocGist, that could be just enough.)
I think embedding AsciiDoc documents into other tools this way would be great.

Maybe also support resizing the iframe to fit the document. See:
https://github.com/davidjbradshaw/iframe-resizer

[nawroth]

We could just hidde the navbar but keep the footer once we have one (see #23).

[nawroth]

Example: http://gist.asciidoctor.org/?4f5b891006d0ca947892