Appendix Labels: Digits in headers

* Splits up the xml parsing files into appendices + interpretations
* Accounts for appendix headers like A-2 and B-3(a)

This is the first step towards more intelligent appendix labels. Next steps:
1. Account for headers of the form "Part I" and "Part II"
2. Catch some missing headers (which are in P tags)
3. Account for parentheses-based paragraph markers (in Ps and HDs)
4. Account for decimal-based paragraph markers (in Ps and HDs)