feat(select): ngOptions support disabling options

Extend the ngOptions DSL to support disabling options. Since <option>
elements are generated by ngOptions, we cannot use ngDisabled in this
scenario. Via extend the DSL, user have control over if a option
is diabled on arbitrary condition.

This commit insert a `disable if cond` match syntax into
NG_OPTIONS_REGEXP, change other match index accordingly, add disabled
attribute to null option element and newly cloned option elements,
add 2 new unit tests, and change docs as well.

Closes #638