nesting like user__first_name

[shalakhin]

@and3rson I think it would be great if

widgets = {
    'user': SearchableSelect(model='users.User',
                             search_field='email',
                             many=False, limit=10),
}

will be

widgets = {
    'user': SearchableSelect(model='users.Student',
                             search_field=['user__email', 'user__first_name', 'user__last_name'],
                             many=False, limit=10),
}

If you agree I will make it.

[and3rson]

This implementation assumes #9 as well, so I think those two improvements can be joined here.

[and3rson]

Hey!
Sorry for the huge delay. I've recently been busy with a lot of other stuff.
The project is currently looking for maintainers.
Please open a issue if you are willing to contribute some time to this.
Thanks!