feat(md-collection): add element for secondary-content

src/collection/collection-item.html

@@ -1,4 +1,5 @@
 <template class="collection-item">
   <require from="./collection-item.css"></require>
   <content></content>
+  <content select="md-collection-item-secondary"></content>
 </template>

src/collection/md-collection-item-secondary.html

@@ -0,0 +1,3 @@
+<template class="secondary-content">
+  <content></content>
+</template>

src/config-builder.js

@@ -109,6 +109,7 @@ export class ConfigBuilder {
   useCollection() : ConfigBuilder {
     this.globalResources.push('./collection/collection');
     this.globalResources.push('./collection/collection-item');
+    this.globalResources.push('./collection/md-collection-item-secondary.html');
     return this;
   }