option to not surround header in atx hashes

[blaenk]

It'd be nice if we could choose not to surround a header with atx hashes, e.g.

## this is my header

instead of

## this is my header ##

However, markdown-mode seems to force it as you can see on this line.

[jrblevin]

I've noted this as a feature request.

[blaenk]

Thanks!

[jonmountjoy]

See #32, which provides this functionality.