Add layerName option for WMSGetFeatureInfo format

This option allows the format to read only features of a
given layer. This is useful if you wish to make a single
query to a WMS server with multiple layers in it.

examples/getfeatureinfo-layers.html

@@ -0,0 +1,28 @@
+---
+layout: example.html
+title: WMS GetFeatureInfo (Layers)
+shortdesc: >
+  Shows how to fetch features per layer name in a single WMS GetFeatureInfo
+  request
+docs: >
+  Demonstrates the use of the `layers` option in the
+  `ol.format.WMSGetFeatureInfo` format object, which allows features returned
+  by a single WMS GetFeatureInfo request that asks for more than one layer
+  to be read by layer name.
+resources:
+  - https://code.jquery.com/jquery-1.11.2.min.js
+---
+<table id="info">
+  <tr>
+    <td>All features:</td>
+    <td id="all"></td>
+  </tr>
+  <tr>
+    <td>Hotel features:</td>
+    <td id="hotel"></td>
+  </tr>
+  <tr>
+    <td>Restaurant features:</td>
+    <td id="restaurant"></td>
+  </tr>
+</table>