Added possibility to use option groups for dropdown select
* add tests for passing value / label. * add default value if val === 0
add test for optgroups with callback
test for optgroups with collections
* refactor rendering of options
* add tests
* support option groups with any of the other format of supporting the options
* allow each option groups to be specified as its own callback
add test for mixing options of optgroups in select dropdowns
remove unused code
fix test
Add ui test example with option groups
Add contributor notes
Build