Don't customize :escaped_chars span parser. This will fix part of the issues where we get escaped brackets and colons in at kramdown files.
Updated code documentation, removed obsolete code
Added ImportWhitespaceSanitizer for consistent handling of whitespace during folio and idml imports.
Refactor Kramdown::Parser::Folio so that we can call #parse_to_kramdown_document for round-trip kramdown tree comparisons in validation.
Merge adjacent elements again after doing the other tree post-processing steps as we may have new identical siblings at this point.
Added Kramdown::Element#compare_with for round-trip kramdown tree comparisons in validation.