Support collapsing of markdown sections

Creator supports collapsing of code comments. Recently it seems to have started highlighting markdown files too. It would be awesome if it could collapse sections of markdown files denoted by headers.

For example, you could collapse the "Blah" header:

## Blah

Blah blah

## Blah Blah blah

Blah blah blah blah

to this:

## Blah
[...]
## Blah Blah blah

Blah blah blah blah

This would make editing and navigating larger markdown documents really easy.