feat(sanitize): Allow sanitize strategy defined as a service

 This change allows to define a strategy like this one:

 ```js
 app
 config(function($translateProvider, $translateSanitizationProvider) {
   $translateSanitizationProvider.addStrategy('myStrategy', 'myStrategyService');
   $translateProvider.useSanitizeValueStrategy('myStrategy');
 })
 .factory('myStrategyService', function ($sce) {
   return function (value, mode) {
     if (mode === 'text') {
       return 'something in text mode';
     }
     return 'something in params mode'';
   };
 })
 ```

src/service/sanitization.js

@@ -140,12 +140,24 @@ function $translateSanitizationProvider () {
    */
   this.$get = function ($injector, $log) {
 
+    var cachedStrategyMap = {};
+
     var applyStrategies = function (value, mode, selectedStrategies) {
       angular.forEach(selectedStrategies, function (selectedStrategy) {
         if (angular.isFunction(selectedStrategy)) {
           value = selectedStrategy(value, mode);
         } else if (angular.isFunction(strategies[selectedStrategy])) {
           value = strategies[selectedStrategy](value, mode);
+        } else if (angular.isString(strategies[selectedStrategy])) {
+          if (!cachedStrategyMap[strategies[selectedStrategy]]) {
+            try {
+              cachedStrategyMap[strategies[selectedStrategy]] = $injector.get(strategies[selectedStrategy]);
+            } catch (e) {
+              cachedStrategyMap[strategies[selectedStrategy]] = function() {};
+              throw new Error('pascalprecht.translate.$translateSanitization: Unknown sanitization strategy: \'' + selectedStrategy + '\'');
+            }
+          }
+          value = cachedStrategyMap[strategies[selectedStrategy]](value, mode);
         } else {
           throw new Error('pascalprecht.translate.$translateSanitization: Unknown sanitization strategy: \'' + selectedStrategy + '\'');
         }

test/unit/service/sanitization.spec.js

@@ -190,6 +190,35 @@ describe('pascalprecht.translate', function () {
     });
   });
 
+  describe('$translateSanitization', function () {
+    var $translateSanitization;
+
+    beforeEach(module('ngSanitize'));
+    beforeEach(module('pascalprecht.translate', function ($provide, $translateSanitizationProvider) {
+      $provide.factory('mySanitizeService', function () {
+        return function (value, mode) {
+          return '1' + value + '2' + mode;
+        };
+      });
+      $translateSanitizationProvider.addStrategy('mySanitize', 'mySanitizeService');
+      $translateSanitizationProvider.useStrategy('mySanitize');
+    }));
+    beforeEach(inject(function (_$translateSanitization_) {
+      $translateSanitization = _$translateSanitization_;
+    }));
+
+    describe('should allow specifying a different strategy which is the alias of an existing service', function () {
+      it('for text', function () {
+        expect($translateSanitization.sanitize('Donald <strong>Duck</strong>', 'text')).toEqual('1Donald <strong>Duck</strong>2text');
+      });
+
+      it('for params', function () {
+        expect($translateSanitization.sanitize('Donald <strong>Duck</strong>', 'params')).toEqual('1Donald <strong>Duck</strong>2params');
+      });
+    });
+
+  });
+
   describe('$translateSanitization#sanitize without ngSanitize', function () {
     var $translateSanitization;