Added Node#previous_element and Node#next_element.

These methods can be used similar to #previous and #next expect that they only
return Element instances opposed to all Node instances.

lib/oga/xml/node.rb

@@ -80,6 +80,36 @@ module Oga
         return index <= length ? node_set[index] : nil
       end
 
+      ##
+      # Returns the previous element node or nil if there is none.
+      #
+      # @return [Oga::XML::Element]
+      #
+      def previous_element
+        node = self
+
+        while node = node.previous
+          return node if node.is_a?(Element)
+        end
+
+        return
+      end
+
+      ##
+      # Returns the next element node or nil if there is none.
+      #
+      # @return [Oga::XML::Element]
+      #
+      def next_element
+        node = self
+
+        while node = node.next
+          return node if node.is_a?(Element)
+        end
+
+        return
+      end
+
       ##
       # Returns the root document/node of the current node. The node is
       # retrieved by traversing upwards in the DOM tree from the current node.