Configurable Markdown extensions

This lets the YAML configuration specify a list of Python Markdown extensions
to enable (on top of the default set). This is especially handy, for example,
when you want nice typography via the SmartyPants extension.
Add docs for markdown_extensions
Fix crash when markdown_extensions is not set

This lets the value be a tuple as well as a list. (Never really understood why
Python won't concatenate lists and tuples.)
Add test for extension enabling

I didn't use the SmartyPants extension here because the version of
python-markdown pinned here (2.3.1) doesn't include the extension. 2.4 does;
maybe the project should consider updating this dependency.
Add test for duplicated custom extension names
Fix comment copypasta error
Merge pull request #74 from sampsyo/master

Configurable Markdown extensions