docs(): add usage tag

Closes #685

scripts/docs/tag-defs/tag-defs.js

@@ -1,3 +1,4 @@
 module.exports = [
   {'name': 'demo'},
+  {'name': 'usage'}
 ];

scripts/docs/templates/common.template.html

@@ -115,6 +115,13 @@ docType: "<$ doc.docType $>"
 <@ block description @>
 <$ doc.description | marked $>
 <@ endblock @>
+
+<@ if doc.usage @>
+<@ block usage @>
+<$ doc.usage | marked $>
+<@ endblock @>
+<@ endif @>
+
 <h1 class="class export"><$ doc.name $> <span class="type"><$ doc.docType $></span></h1>
 <p class="module">exported from {@link <$ doc.moduleDoc.id $> <$doc.moduleDoc.id $> }<br/>
 defined in <$ githubViewLink(doc) $>
@@ -155,7 +162,7 @@ defined in <$ githubViewLink(doc) $>
 <@ endfor -@>
 <@- endif -@>
 
-<@ if doc.properties @> 
+<@ if doc.properties @>
   <h2>Attributes:</h2>
   <table class="table" style="margin:0;">
     <thead>