Improve sanitization options: making it pluggable

I would like to have more sanitization options for raw html in markdown. For example: i would like to allow raw html bits like <table>, <tr> and </table> but disallow evil things like <script>takeOverTheWorld();</script>. Right now: we can either allow or disallow all raw html. If we make the sanitation pluggable, the user can choose which sanitation library he/she wants to plug in.

The most straightforward way to make sensitization pluggable would be to provide a function that does what 'potentiallyUnsafe' in lib/html.js does, plus a sanitizeRawHtml function that could be used for HtmlInline and HtmlBlock instead of just always omitting a comment '<!-- raw HTML omitted -->'.

This would be a minimum modification from what we have. I will create a pull request for this.

Read lib/html.js, especially potentiallyUnsafe, and trace how HtmlInline and HtmlBlock currently handle raw HTML. Define the sanitizer hook and replacement behavior described in the issue, then verify that safe tags such as table, tr, and /table can be retained while script content is rejected.