Add space in header regex

[lggruspe]

Fixes #597

Also adds some tests for HeaderDecrease and HeaderIncrease.

[tomtomjhj]

I'm in favor of this change, but I'd like to hear from another reviewer, since some markdown implementations don't require spaces. https://babelmark.github.io/?text=%23asdf

[lggruspe]

We could add an option to toggle this behavior. Maybe g:vim_markdown_hashtags or g:vim_markdown_space_in_atx_header will do?

What do you think, @tomtomjhj?

[alerque]

See comment in #604 (comment) that covers why we don't know whether this should be merged directly or not. The discussion about what Markdown flavor we are even targeting needs to be resolved first.

[alerque]

Also just for the record these two PRs are not duplicates, one addresses the syntax rules and one addresses the header level handling rules. Both should be merged, modified, or rejected in the same way.