bash completion for `brew list --multiple`

As per 9ab605c7f1883 removal of `brew dirty`. `--multiple` *implies* `--versions`, thus IMO it shouldn't be necessary to also explicitly include the `--versions` option, but that's a separate matter. For now this offers the small added convenience of:

    $ brew list --m<TAB><TAB>

to quickly expand the `brew dirty` equivalent.