GRIT: Enable variable expansion in filenames during HTML inlining.

grit/gather/interface.py

Index: grit/gather/interface.py
===================================================================
--- grit/gather/interface.py	(revision 124)
+++ grit/gather/interface.py	(working copy)
@@ -152,6 +152,10 @@
     '''
     pass
 
+  def SetFilenameExpansionFunction(self, fn):
+    '''Sets a function for rewriting filenames before gathering.'''
+    pass
+
   # TODO(benrg): Move this elsewhere, since it isn't part of the interface.
   def _LoadInputFile(self):
     '''A convenience function for subclasses that loads the contents of the