Add folding to header elements

[cloverich]

When my notes get long I regularly want to keep working in the same note, but the clutter and need to scroll makes it difficult. It would be great to have code folding enabled at the heading level and a command to folder all / unfold all / fold current.

Having the toolbar float down, appear on hover, or have better shortcuts (#103 ) might alleviate this need to an extent. But for bigger notes (very common) it is still ideal to be able to fold / unfold as a means to browse back and forwards, particularly as you organize the note.