Kramdown use enable_coderay instead of use_coderay for syntax highlighting

By default, Jekyll set use_coderay to false while Kramdown set enable_coderay to true, so Coderay will be always enabled although use_coderay is set false.

This patch fixed the bug without losing compatibility.

ref: http://kramdown.gettalong.org/options.html
Compatibility goes into `Configuration` – we take the input and ensure the old values have the right keys. My other question is: when did this change in Kramdown?