This makes the terminal go really funkie and sometimes crash when
printing the error messages to the current process stdout where nvim is
running.
Now the error output is sent to the `popen` output and displayed in the
ivy completion buffer without messing with the current process io.
Now you can open the currently selected item in a vertical split or a
horizontal split. The completion callback must support the current
actions.
There is also a bit of testing in here. The vim mock has been refactored
and split out so we can use it multiple tests.
