Use go.net/html's parser to sanitize HTML.

Use an HTML5 compliant parser that interprets HTML as a browser would to parse
the Markdown result and then sanitize based on the result.
Escape unrecognized and disallowed HTML in the result.
Currently works with a hard coded whitelist of safe HTML tags and attributes.
Merge pull request #69 from mprobst/master

Use go.net/html's parser to sanitize HTML.