common: Rename formatter.hpp -> highlight.hpp.

Better name, and matches scripts/highlight.
diff --git a/scripts/highlight.py b/scripts/highlight.py
index 9c12e82..ed3b54d 100644
--- a/scripts/highlight.py
+++ b/scripts/highlight.py
@@ -1,4 +1,4 @@
-##########################################################################
+#########################################################################
 #
 # Copyright 2011-2012 Jose Fonseca
 # Copyright 2008-2009 VMware, Inc.
@@ -31,7 +31,7 @@
 
 
 class PlainHighlighter:
-    '''Plain formatter'''
+    '''Plain highlighter.'''
 
     black = None
     red = None