Merge changes between this repo and xonsh repo

[anki-code]

This PR is the result of merging current versions of xonsh/xonsh/completers/bash_completion.py and py-bash-completion/bash_completion.py.

[anki-code]

Hi @gforsyth!
I've merged two versions of one file and to avoid this mess in the future I suggest to remove/archive this repository with moving the result code to xonsh/completers/bash_completion.py.
If we want to have this distinct repo please describe why. I suppose we wanted to provide this functionality as a library but in this case we need to create a xontrib with package and add it to xonsh repository as dependency - this looks like the right way. So let's do either removing/archiving it or create a xontrib and remove it from xonsh repo.