Disabled options should be shown and not selectable

@pfiller This is addressing Issue https://github.com/harvesthq/chosen/issues/488.

**Assumptions**

* Functionality for options to be disabled (not optgroups)
* Disabled options should be shown in search results, just not selectable

**Remaining issues:**

- [ ] Disabled items do not correctly filter out when searching
- [ ] Update styling of disabled items

cc: @stof