add option to click on group of items

[jfernandogt]

if selectGroup is true you can click in a group and select all of options of that

[arnold-almeida]

How does this look like?

The Optgroup Support example found here is pretty neat...

http://hemantnegi.github.io/jquery.sumoselect/sumoselect_demo.html

[sbauch]

I'm using this in a project, think some styling could be improved, but curious what the maintainers think about getting this merged.

@jfernandogt would you take PRs on your branch for some of the styling stuff? i.e. I'd like to see the same hover state on Groups that are on Options, and I also think the options hover should extend to the full width of the dropdown.

But maybe that's best resolved by my own styles?