Configure Markdown engine

The README says that [the Markdown rendering engine can be selected in the configuration file][1], but the call to process this config option has been commented out in the current version. Going through the code and the commit that made the change, I cannot see why. So I reinstated it so that I could use Bluecloth/Discount, and it works fine. Except...

I wanted to change the default options that Bluecloth passes to Discount, and I figured that I could use the same mechanism as you used for configuring Maruku (adding a config key with the name of the Markdown engine).

However, Bluecloth expects the hash keys to be symbols, and I didn't see a way to generate those with the current JSON parser. But... I did find an suggestion for converting hash keys from strings to symbols, which seemed like a good way to go --- I can't think of a case where that wouldn't be as good or preferable. I updated the Maruku config code to use symbols instead, but I haven't got Maruku installed so I haven't tested that it still works.

[1]: https://github.com/schacon/showoff#markdown-engine