add XSL script to extract data from EAD files.
rename XML output element 'letters' to 'elements'

This makes the data holder element more generic, as we can now also include
other struture elements e.g. pictures or maps.
adds id option to dump volume information for that id

* make ID option required
* refactor extraction and print
* refactor extraction and transformation methods
* move XSLT processing to convenience class
* use variable resolver instead of concatenating unquoted strings
* moved XSLT transformation to separate class
* adds mutually exclusive option group