Add 'slug' argument to BP_Groups_Group::get().

[dcavins]

I'd like to add the ability to find groups by slug using groups_get_groups(). I'm planning on using this functionality to solve other problems, so have developed patches in other tickets. But it makes sense for a parameter change to have its own ticket for ease of finding it later.

Attached is the most recent version of the patch as originally developed to support work in #7477. This is ready to go in and has received useful feedback in #7477. I'll commit it later today.

[dcavins]

Add 'slug' parameter to BP_Groups_Group::get().

[dcavins]

In 11523:

Add 'slug' parameter to BP_Groups_Group::get().

Add $slug parameter to groups_get_groups() and its underlying
function BP_Groups_Group::get(). Find a group or groups by passing a
single slug, a comma- or space-separated list of slugs, or any array of
slugs.

Fixes #7496.