v0.25.2: Markdown folding improvements

More improvements for markdown folding!

Tweaks

• Added shortcuts for folding specific heading levels for sublime and emacs key bindings.
• Added toggle fold command for default key bindings
• Cursor can now move past fold markers without clearing them
• Prevent folding when no lines would be folded.
• Deleting a fold marker (via delete key) now unfolds the content instead of deleting whole region, similar to backspace key.
• Unfold and toggle fold commands now work when used on unfolded content that follows a fold marker, even if that content is technically on a different line.

Bugfixes

• Fixed first heading not being foldable if it was also the first line in the document
• Fixed repeatedly folding the same line creating nested fold markers that hade to be cleared multiple times
• Fixed one cause of repeated fold markers appearing on the same line
• Fixed more than three periods at the end of a heading getting folded as well (folding is now limited to the last three periods).
• Fixed fold marker getting deleted prematurely when backspacing
• Fix deleting characters on lines after folds sometimes unfolding above lines
• Workaround (read: hack) for cursor not appearing next to fold markers. Still occurs in one case
• Fixed deleting a fold marker sometimes unfolding a nested marker as well