[WIP] Whitelist ID attributes in heading and a tags

To allow the use of anchor links within the editor we would need to either specify an anchor using `<a id=“foo”></a>` or link to an element with an `id` attribute. marked.js automatically adds IDs to heading tags so we'd get that for free, but at the cost of every heading having an ID (currently these are stripped out by scribe). To link to the anchor in Markdown we would use `[foo](#foo)`.

@dougdroper @tomas-stefano - What are your thoughts on this way of doing it this way? Should we enable IDs on all headings? Or just force the user to add an anchor tag?