Maruku fenced code blocks

This adds support for the `fenced_code_blocks` option in Maruku, and adds a test that verifies that it works.

**Note**: This test accommodates for the newlines that Maruku 0.7.0 currently puts around the internals of `<code>` blocks generated by `fenced_code_blocks`. If my PR fixing this at bhollis/maruku#112 is merged in, and Maruku's version is bumped, I'll follow up with another PR that bumps Jekyll's Maruku version to it, and removes the newlines from this test.