Now when trying to open files and buffers it will use the `buffer`
command when there is an existing buffer with the same name. This will
allow us to open non file buffers like terminals and log buffers.
By using the `bufnr` function to get the existing buffers, it will also
work for buffers that have been renamed with `title` command
Ref: #31
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.
