Adding support for capturing raw attribute value strings (vsrc), and for an --inputfile option for parse test as an alternative to stdin

Amend by gwicke:
- Only escape entities in the serializer when an attribute value is not
  round-tripped from source.
- Comment out a stray debug print

Overall, 10 additonal wt2wt tests are passing with this patch.

Change-Id: Ia26be4af360bd67926170cabe1a76cb20e007475