Refactor expand node differ

[geohci]

• Support detailed diffs for HTMLEntities
• Unrecognized HTML tags are just categorized as Other Tag
• Improve text formatting handling:
  • Treat text formatting like templates -- i.e. changes to type and/or contents trigger a change (as opposed to just type)
  • HTML tags are now treated case-insensitively in line with official parser so e.g., <Small> is the same as <small> -> Text Formatting
  • Formatting is context-dependent so preserve original tag to avoid parsing errors later on in processing
  • Begin incorporating mwconstants external library in place of edit-types-maintained constants

Closes #75 and addresses parts of #74 .