Added automatic list-enumeration upon addition or deletion of items + some refactors

[nmder]

Hi there,

The key features added are:

1. Deleting, pasting, or adding numbered items in normal mode---with d, p, and o/O, respectively---automatically adjusts the indices.
2. Shift-Tab to de-indent bullets through <Plug>Markdown_DeindentListItem.

Also, vim.on_key callback is now restricted to post item-insertion only.

Thanks!