Fix automatch * when closing bold

Suppose you want to write a bold word.
You type in `**Word|` (here `|` is the cursor) and you now want to close the `**`.
You press `*` and you have `**Word*|` but then when you press `*` again, it gets automatched resulting in `**Word**|*`. You then have to advance the cursor and delete the extra `*`.
Obviously you could also select `Word` and press `*` twice but the other workflow should also be supported.

This PR fixes the problem by disabling automatch of `*` when the preceding text is a letter or `*`.