feat: deprecate custom decorators

the function "registerDecorator" is now deprecated. The
decorator API exposes a lot of internal structures that
should not be part of the official API. An alternative
API can be discussed at #1574
- "Handlebars.registerDecorator" now logs a warning
- "Handlebars.registerDecoratorWithoutDeprecationWarning"
  can be used in order to omit the warning, when
  developers have read the notice.
  (The inline-decorator is using this function now)

lib/handlebars/base.js

@@ -63,13 +63,19 @@ HandlebarsEnvironment.prototype = {
   },
 
   registerDecorator: function(name, fn) {
+    this.log('warn', 'Custom decorators are deprecated and will be removed in the future. Join the discussion at https://github.com/wycats/handlebars.js/issues/1574');
+    this.registerDecoratorWithoutDeprecationWarning(name, fn);
+  },
+
+  registerDecoratorWithoutDeprecationWarning: function(name, fn) {
     if (toString.call(name) === objectType) {
       if (fn) { throw new Exception('Arg not supported with multiple decorators'); }
       extend(this.decorators, name);
     } else {
       this.decorators[name] = fn;
     }
   },
+
   unregisterDecorator: function(name) {
     delete this.decorators[name];
   }

lib/handlebars/decorators/inline.js

@@ -1,7 +1,7 @@
 import {extend} from '../utils';
 
 export default function(instance) {
-  instance.registerDecorator('inline', function(fn, props, container, options) {
+  instance.registerDecoratorWithoutDeprecationWarning('inline', function(fn, props, container, options) {
     let ret = fn;
     if (!props.partials) {
       props.partials = {};

spec/blocks.js

@@ -306,7 +306,7 @@ describe('blocks', function() {
       equals(run, true);
     });
 
-    describe('registration', function() {
+    describe.only('registration', function() {
       it('unregisters', function() {
         handlebarsEnv.decorators = {};