feat(ngOptions): add support for disabling an option

This patch adds support for disabling options based on model values. The
"disable when" syntax allows for listening to changes on those model values,
in order to dynamically enable and disable the options.

The changes prevent disabled options from being written to the selectCtrl
from the model. If a disabled selection is present on the model, normal
unknown or empty functionality kicks in.

Closes #638
Closes #11017