Vim is not thread-safe so posting a message to Vim from a non-GUI thread causes
a crash *sometimes*. I was aware of this problem before, but didn't catch this
instance of it in code review.
Fixes#479.
For instance (`|` represents the cursor):
1. Buffer state: `foo.|bar`
2. A completion candidate of `zoobar` is shown and the user selects it.
3. Buffer state: `foo.zoobar|bar` instead of `foo.zoo|bar` which is what the
user wanted.
This commit resolves that issue.
It could be argued that the user actually wants the final buffer state to be
`foo.zoobar|` (the cursor at the end), but that would be much more difficult
to implement and is probably not worth doing.
Fixes#374.
