Added markdown-insert-list-item

This function will insert new list item (* or number) when you edit a list. By default I bind it to M-<return>, the same way org-mode has it. It is clever about indentation, so it keeps the level previous item is. With `C-u` descend a level, with `C-u C-u` increase a level.

Calls markdown--cleanup-list-numbers-level afterwards to fix the numbering if user added new item in the middle of the list.