Restored disabled markup-engine configuration (ref: 616bf52).

It's not clear to me why this line was commented out, as the commit message says only "whitespace", referring to the removal of extraneous white space from another part of the file. Was it an accident? (e.g., commented out for testing and checked in unintentionally)
Added support for passing options hash from showoff config file to rendering engine via Tilt.

Previously, some options were handled manually for the Maruku engine, but not for any of the others. The markdown engine wrappers in Tilt allow the passing of options to the various libraries through a hash given with the renderer is instantiated. The special handling in Showoff for the Maruku engine have been left unchanged for now, but it might be possible to migrate the code to use the same mechanism in the future. (I haven't used Maruku yet myself, so I didn't want to mess with the config code.)
For the markup engine config settings, hash keys are converted from strings to symbols.

The Tilt markup engine wrappers expect hash keys that are symbols, and there is no way to specify hashes in that format in JSON. But there's no reason the existing Maruku engine settings code couldn't be tweaked to expect symbol keys and work the same way.
Fixes exception when no renderer options specified.

(Oops! Forgot to test with example presentation.)
Merge pull request #194 from MichaelHackett/feature-configure-renderer

Configure Markdown engine