Update Gemfile.lock.
Add file_digest filter.

This allows URL fingerprinting.

The 'site' variable is added to all templates, and site.directory is
added to to_liquid, so that file_digest can work when used in a layout.
Rework file_digest filter into a tag.

Also picks up the site variable from the context, so it isn't needed as
an argument.
Run all tests in production mode.
Ignore trailing whitespace on prefix values.

This was causing a failing test because 'prefix:. %}' was giving a
prefix value of '. '.

An addition test is added to cover this case.
Some stub documentation of file_digest.
Add a new CHANGELOG with file_digest mentioned in it.
Add .md extension to CHANGELOG.
Mention issue number.
Update README.md
Be clearer about ENV requirements in README
Add a simple list of README contents/headings

Thanks, GitHub relative links!
Change underscores in "table of contents" to hyphens
Merge pull request #12 from aprescott/file-digesting

Support URL fingerprinting with a file_digest tag.