Have Meta extension support YAML

The Meta extension currenly only supports extended RFC822-style headers.

Another ubiquitous meta-data format, even prevalent perhaps when it comes to *.md files, is YAML. [Pandoc](http://johnmacfarlane.net/pandoc/) [supports YAML](http://johnmacfarlane.net/pandoc/demo/example9/pandocs-markdown.html#extension-yaml_metadata_block) metadata, [Jekyll uses it](https://help.github.com/articles/using-jekyll-with-pages/#frontmatter-is-required).

It'd be nice if this markdown implementation also supported YAML meta data. Making a plugin is an option, but [the only existing plugin](https://github.com/teoric/python-markdown-yaml-meta-data) seems to not work at all _and_ break the established API.

It'd [make some sense](https://www.google.com/search?q=markdown+metadata) to have YAML meta-data supported out-of-the-box. Please let me know what you think. With heads up, I'll be happy to further write tests and document the addition.
Yes, this is my preference. It makes for easier to read code later and it the way the API is designed to work. If other extensions don't follow this pattern, they should be updated--unless there is a good reason not to of course.