Grouped options support

[josefarias]

Continuing from #87 which I'm closing in favor of this.

I think supporting this makes sense. Open to ideas for what the API should be.

[edwinv]

We are using the grouped_options_for_select Rails helper for populating our current select boxes. This structure works well.

I like the way the current implementation accepts different types of options through ActiveRecord instances, hashes and arrays. It would be great to allow grouping for these types without introducing a new grouped_ helper.

1. For ActiveRecord::Relation types you ideally want to call a method in the instance to fetch the group. This could be a to_combobox_group method.
2. With Single hash, Array of strings and Array of arrays you don't have enough space in the data structure to introduce a group.
3. This leaves us with the Array of hashes, this hash could contain a group value to determine the group.
4. We could introduce a new structure Hash with X, in which X can be any of the above structures. The keys of the hash are the groups to displays. The values are the options to render, which can be a single hash, array of strings and array of arrays.

If the combobox_options helper accepts these structures, the async rendering would also work out of the box.

For rendering the groups, the basic version could render a partial that is not overwritable by the user, only stylable through CSS. For our use case, we don't need custom rendering of groups through the render_in functionality.

[josefarias]

Coming in the next release!

#114