Close even if the target buffer has not been loaded yet

Hello,


When working in split mode, BufExplorer does not close itself if the selected buffer has not been loaded yet. Here is how the behavior can be reproduced:

1. Issue the `vim` command with more than one file (`$ vim one.txt two.txt`).
2. Open BufExplorer in split mode and see that the other buffer is shaded (= not loaded).
3. Open that other buffer and see that it opens right in the split replacing BufExplorer and leaving two windows open. 

I have tested it as much as I could, and it works for me. However, since I am not that familiar with the code base, I could miss some scenario. If it is the case, please let me know, and I will fix the pull request.

Thank you! 


Best wishes,
Ivan