Improved autoformat support; bugfixes

Better behavior for #31, autoformat lists in Markdown

For tpope's markdown syntax, restored standard autoformat behavior for unordered lists, which is generally pretty good.

Ordered lists had annoying behavior for autoformat, so simply disabling autoformat if detected when insert is entered. This isn't ideal, but it's not clear what else can be done now.

Fixed bug for PFormatToggle, where it wasn't properly disabling Vim's autoformat.

Provided help in README for mapping key to toggle autoformat.