Add `googlefile` macro

To render read-only file previews. For example it allows one to embed
a PDF document into a Wiki.

Usage:

    {{googlefile(key, domain, height)}}

Example:

    {{googlefile(1tue9Oj0iMl5Gz752F5rud6LZzo3YjHmqPD7d82cdQyw, example.com, 800) }}

P.S. Along with minor code style fixes.