Add basic bash completion support.

This completes the basic crab commands and their long options.  Needs
to be sourced in the current bash session to work.  Fixes #4331.

To complete the (double) dashed options, at least one dash must be present on the command line.  Usage examples:
```
crab <TAB><TAB>

crab st<TAB> -<TAB><TAB><TAB>
```