Add el option to layers

src/navigator/config/config.js

@@ -1,6 +1,10 @@
 module.exports = {
   stylePrefix: 'nv-',
 
+  // Specify where to render layers on init, string (query) or HTMLElement
+  // With the empty value, layers won't be visible
+  el: '',
+
   // Enable/Disable globally the possibility to sort layers
   sortable: 1,