Exclude xml files from filtering.

I think we should exclude .xml files. Here is a warning for my Phing build.xml file.
```
--------------------------------------------------------------------------------
FOUND 0 ERROR(S) AND 1 WARNING(S) AFFECTING 1 LINE(S)
--------------------------------------------------------------------------------
 1 | WARNING | No PHP code was found in this file and short open tags are not
   |         | allowed by this install of PHP. This file may be using short
   |         | open tags but PHP does not allow them.
--------------------------------------------------------------------------------
```