Bash 3.2 does not have associative arrays, so we hack around that by using
generic varibale names like `opts__$cmd`. We also don't support the "?" alias
anymore.
The command prints a shell script that provides completion for the `beet`
command. To test it run `eval "$(beet completion)"` in your shell.
I also included some crude testing for this. The `test/test_completion.sh`
script runs tests in a shell and exit with a non-zero status code if the tests
fail. It assumes that the completion script is already loaded in the executing
shell.
As of now the completion only works for bash 4.1 and newer.
