Add option to disable all HTML

[cebe]

option to only allow markdown syntax.

[qiangxue]

How about also support disabling/enabling individual features? For example, I may want to disable headers for contents in an article.

[jonathanvaughn]

I think this may be the same feature I'm looking for.

I would like a variation of GFM where any tags outside of code blocks get converted to HTML entities. It would make our use case much easier (we don't really want to have raw HTML/XML code, unless it's in code blocks - so I have to try to pre-filter this myself or trust admins not to enter tags without using inline code blocks or risk breaking page layout).