Add label to denote optional arguments

Prior to this change, the rendered docs didn't indicate which arguments
were optional. This differed from the behavior on atom.io. See

https://user-images.githubusercontent.com/2988/60625840-f1550380-9db6-11e9-821c-d75d31511e15.png

This commit resolves that discrepancy. Optional arguments will now have
an "optional" badge.

lib/filters/api_json_filter.rb

@@ -171,6 +171,7 @@ def argument_row(arg, level)
           #{markdown("`#{arg["name"]}`")}
         </td>
         <td>
+          #{arg["isOptional"] ? '<span class="optional">optional</span>' : ''}
           #{markdown(arg["description"])}
         </td>
       </tr>