Add trait example to index page

Long overdue...considering this is ease.js' most attractive and unmatched
feature.

* scripts/ex/trait.js: New example.
* index.html: Use it.

index.html

@@ -137,6 +137,9 @@
   finalized, but has been included in each GNU ease.js release since v0.2.0,
   and is stable.
 </p>
+<pre class="js">
+  <!--%inc scripts/ex/trait.js-->
+</pre>
 <p>
   Documentation will be available once some final details are
   finalized.  Until that time,

scripts/ex/trait.js

@@ -0,0 +1,57 @@
+const Echo = Class( 'Echo',
+{
+    'virtual public echo'( str )
+    {
+        return str;
+    },
+} );
+
+const Prefix = Trait( 'Prefix' )
+    .extend( Echo,
+{
+    'private _prefix': '',
+
+    __mixin( prefix )
+    {
+        this._prefix = ''+prefix;
+    },
+
+    'public abstract override echo'( str )
+    {
+        return this._prefix + this.__super( str );
+    },
+} );
+
+const Suffix = Trait( 'Suffix' )
+    .extend( Echo,
+{
+    'private _suffix': '',
+
+    __mixin( suffix )
+    {
+        this._suffix = ''+suffix;
+    },
+
+    'public abstract override echo'( str )
+    {
+        return this.__super( str ) + this._suffix;
+    },
+} );
+
+const UpperCase = Trait( 'UpperCase' )
+    .extend( Echo,
+{
+    'public abstract override echo'( str )
+    {
+        return this.__super( str ).toUpperCase();
+    }
+} );
+
+// stackable, parameterized traits
+Echo.use( Prefix( "Bar" ) )
+    .use( Suffix( "Baz" ) )
+    .use( UpperCase )
+    .use( Prefix( "Foo" ) )
+    .use( Suffix( "Quux" ) )().echo( "Inner" );
+
+// result: FooBARINNERBAZQuux