Expressify provides:
It is packaged in a single file suitable for use as a Jekyll plugin but the core expression evaluator can be used independently.
Canonical repository: http://gitorious.org/expressify
Copyright © 2013, Hugues Bruant hugues@bruant.info
Licensed under 2-clause BSD. See bsd.txt for details.
Simply copy expressify.rb
to the plugins dir of your Jekyll site.