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.
Dunno. Inspecting all the built-in extensions, I've seen examples of both, and more. This way, we avoid one thin `__init__()`. Since this isn't functional programming and since `MetaPreprocessor` and `MetaExtension` are tangled one way or another, doesn't really matter IMHO. Unless you have a preference. ¯\\\_(ツ)\_/¯