Improve performance of tag queries.

Read the chapter on MongoDB in "Seven Databases in Seven Weeks", noticed
that there's a way to query within array fields, and that you can bring
back only the fields you want.

There's probably ways to make it even quicker, but this'll do for now.
Sanitize generated HTML to prevent XSS attacks.

Even though I'm currently the only person posting content on the blog,
it might not be a bad idea to sanitize the generated HTML at this point.

Links:
 - http://www.troyhunt.com/2010/05/owasp-top-10-for-net-developers-part-2.html
 - http://michelf.ca/blog/2010/markdown-and-xss/
 - https://github.com/theSmaw/Caja-HTML-Sanitizer