Ordered list numbering becomes off if an item is inserted in middle of list

[AndrewSverdrup]

First off, thanks for making this awesome markdown editor!

When creating an ordered list and typing one item at a time, the list items are automatically numbered. For example:

1. foo
2. bar
3. baz

But then if I add an item in the middle of the list I get:

1. foo
2. qux
2. bar
3. baz

The markdown parser still renders it correctly, but in the editor view the list item numbering is not updated.

Similar to #563