Added inline gist for Markdown, refactored markdown extension handling.

1. Added a Markdown extension which adds a "gist" directive similar to the reStructuredText one added recently.
1. Refactored custom Nikola post-processing of Markdown-rendered HTML into a Markdown plugin.

The [Python Markdown Extensions API](http://pythonhosted.org/Markdown/extensions/api.html#makeextension) states that "Each extension should ideally be placed in its own module starting with the `mdx_` prefix."  I used this naming convention, but placed the modules under `nikola.plugins.compile_markdown` rather than have Nikola install modules outside the `nikola.*` namespace.  This prevents the `markdown.markdown()` call from resolving these extensions by name, so I use instances of the extension classes instead.