Add minimal bash support

[erikw]

This PR adds support for bash, that works along-side (an unaffecting) the zsh support. Completion for bash is added as well, including relative path completion ($ cd-bookmark -c BOOKMARKID/path<tab>)!

This patch was tested on zsh 5.8 and bash 5.1.8 on macOS and it works with both shells.

This is a minimal support patch. I can understand if you don't want to merge this PR as-is, as maybe you would like to structure it differently. In that case, let's use this PR as a start of the conversation.

Otherwise, this PR does add working bash support without compromising zsh support. I've encountered no issues so far.