Use redcarpet as the default markdown engine

It is closer to the GitHub syntax, and is what most people using GitHub and Jekyll are most familiar with.

Also use some extensions:

- smart: Enable SmartyPants. kramdown had this on by default
- tables: Enable table parsing. kramdown had this on by default
- with_toc_data: Enable heading anchors. All markdown documents on GitHub have this on, so why not here too?