Support for disabling Sanitization

While sanitization is an important feature of a markdown parser, it is sometimes useful to disable it. For example, on Discourse we have our own sanitizer that is executed further down the pipeline after the markdown has been rendered, so we needn't execute the sanitization twice. Also, ours is a little more flexible and allows particular tags to be whitelisted.

This patch supports an option, `sanitize: false`, when rendering, to not perform sanitization. This option is not set by default so the existing behaviour stays the same unless expressly disabled.

Note this patch is based on the src attribute PR because it includes two rendering tests which belong to a newly introduced file in that PR.