Add readonly support

[nicklasb]

How should this work?
Should it be possible to drop down and select?

[chargrove]

@nicklasb It appears that both ui-select and angular-strap select support the disabled attribute. We should be able to support both using ng-disabled.

The behavior on each is to show selected options (or the initial placeholder), but not to allow dropdown or selection/removal.

[nicklasb]

@chargrove To me, disabled means greyed out as well, however I won't bikeshed on that. All for it.

[albertjose]

@nicklasb Why disabled don't work in strapselect? I modified source code of strapselect.html adding ng-disabled="form.disabled" and it worked.

[nicklasb]

Please, I am for it, a PR would be great.