feat($compile): allow directives to modify interpolated attributes

A directive can now set/update/remove attribute values even those containing
interpolation during the compile phase and have the new value be picked up
during the compilation.

For example in template:

<div replace-directive some-attr-or-directive="{{originalInterpolationValue}}"></div>

the replace-directive can now replace the value of some-attr-or-directive during compilation
which produces this intermitent template:

<div replace-directive some-attr-or-directive="{{replacedInterpolationValue}}"></div>

or even

<div replace-directive some-attr-or-directive="replacedStaticValue"></div>

as well as

<div replace-directive some-attr-or-directive></div>

test/ng/compileSpec.js

@@ -1503,6 +1503,12 @@ describe('$compile', function() {
           expect(attr.$observe('someAttr', observeSpy)).toBe(observeSpy);
         };
       });
+      directive('replaceSomeAttr', valueFn({
+        compile: function(element, attr) {
+          attr.$set('someAttr', 'bar-{{1+1}}');
+          expect(element).toBe(attr.$$element);
+        }
+      }));
     }));
 
 
@@ -1544,6 +1550,14 @@ describe('$compile', function() {
     }));
 
 
+    it('should allow directive to replace interpolated attributes before attr interpolation compilation', inject(
+        function($compile, $rootScope) {
+      element = $compile('<div some-attr="foo-{{1+1}}" replace-some-attr></div>')($rootScope);
+      $rootScope.$digest();
+      expect(element.attr('some-attr')).toEqual('bar-2');
+    }));
+
+
     it('should call observer of non-interpolated attr through $evalAsync',
       inject(function($rootScope, $compile) {
         $compile('<div some-attr="nonBound" observer></div>')($rootScope);