Import changes from Lanli (HTML comment parsing)
Expose targetted CommonMark version
Make ATX headers compliant
:sparkles: Now targeting CommonMark 0.13!
API: Improve pool API
API: Make document use the new API
API: Parser always pops objects now
API: Changes in render API

Instead of output, the parameter is now named request
and it's the last parameter in hoedown_document_render.

The request parameter is now mainly a per-render input
parameter, that can be accessed at any time via data->request.

render_end now directly returns the output, which in turn
is returned by hoedown_document_render to the user.
This makes for a simpler API.

All global callbacks are required now.
API: Use the new API
DROP current emphasis implementation
new-emphasis: An "emphasis stack" is maintained
new-emphasis: Stack-based implementation!!
new-emphasis: Make renderers use the new API
Strip leading line spaces too
Fix flags
Make it score better when testing without normalization