move sanitize to its own exporter option

[janetriley]

export.py's --scheme argument accepts json and html, as well as sanitize levels raw, safe, and text.

Move sanitize levels to their own argument and ensure they get passed in properly to Post.htmlize().

[janetriley]

What should the single-letter flag be? I was thinking of --sanitize but -S is already taken by scheme. --level and -L don't seem intuitive enough.

[bbengfort]

Hmm, this is a tricky one. Would it be too weird to do --sanitize and -L? What about --sanitize and -z?