Do not highlight or be able to select disabled options

@pfiller @stof This is my second attempt at showing disabled options in stead of removing them ([first attempt](https://github.com/harvesthq/chosen/pull/1173)).

This version works to filter out disabled options when searching.

*Expectations:*

- [x] Disabled options are 'grayed out'
- [x] Disabled options filter away when not match on searching
- [x] Disabled options displayed 'grayed out' when matched on searching
- [x] Disabled options do not highlight when hovering
- [ ] Disabled options do not highlight when using arrow keys
- [x] Disabled options do not highlight when matched on searching

*Open issues/questions:*

- This does not handle disabled optgroups
- What is the expectation when option is disabled AND selected?