WIP: Updated Folio import files:

* commandline: 
    * persist additional output sinks
* Kramdown::Parser::Folio 
    * use Kramdown doc instead of string concatenation
    * change API to accept XML string instead of file name
    * generate deleted_text, editors_notes and warnings in addition to AT
* Kramdown::ElementFi - subclass kramdown's Element for additional behavior.
Added script to generate element inventory from single XML file.
Updated Folio importer