rzSliderTplUrl should be marked as optional.

[mickdelaney]

Steps to reproduce

1. Turn on strict Component Bindings $compileProvider.strictComponentBindingsEnabled(true);
2. Render an rzslider without setting the rzSliderTplUrl

   
   <rzslider
   rz-slider-model="$ctrl.sliderModel"
   rz-slider-options="$ctrl.sliderOptions">
   </rzslider>

3. 
Error: [$compile:missingattr] Attribute 'rzSliderTplUrl' of 'rzslider' is non-optional and must be set!
https://errors.angularjs.org/1.7.5/$compile/missingattr?p0=rzSliderTplUrl&p1=rzslider
    at angular.js:138
    at strictBindingsCheck (angular.js:11243)
    at initializeBinding (angular.js:11266)
    at forEach (angular.js:395)
    at initializeDirectiveBindings (angular.js:11255)
    at nodeLinkFn (angular.js:10472)
    at angular.js:10900
    at processQueue (angular.js:17914)
    at angular.js:17962
    at Scope.$digest (angular.js:19075)

### Expected behaviour
Tell us what should happen
Given the value is actually optional (there's a default value of 'rzSliderTpl.html'
It should be marked as @? (optional)

### Actual behaviour
Its not optional

[ValentinH]

Hi, I didn't know about strictComponentBindingsEnabled. Would you like to submit a PR fixing this?

[mickdelaney]

Ok, leave it me with.