No difference between options in optgroup and other options

[Inkbug]

Currently, one can't differentiate between options inside an optgroup, and options outside of an optgroup. Would it be possible to indent the options in an optgroup, or at least create some classes so one can indent it on one's own?

See also #135.

[wenzhixin]

Example: http://jsfiddle.net/wenyi/knbdm5hu/11/