Editable sections.

[dometto]

PR to discuss pros/cons of this experimental implementation of gollum/gollum#715

Another way of implementing this would be by parsing the document into sections client-sided, through the markup libraries already available as part of the ACE editor. However, I don't know if any them support extracting sections, and the problem remains of how to map id's generated by gollum's TOC filter to the raw markup.

[dometto]

We're now considering moving to markdown.js for the livepreview (gollum/gollum#826). markdown.js supports generating an intermediate representation of the data, which we could possible use to split the sections up. We could look up the correct id's for each section via an ajax call. Possible pitfall is the fenced codeblocks: since markdown.js doesn't support these (yet -- evilstreak/markdown-js#41), lines starting with # in a code block will probably be misinterpreted as section headings.

EDIT: markdown.js can't reconstruct markdown out of jsonml, so we'd have to hack the parser to include the linenumbers of each section in the jsonml, much like the kramdown hack in this PR.