Support whitelisting attributes, which will be ignored for diffing but preserved in the output

[caffodian]

A lot of changes result in fairly ugly diffs since we only represent diffs as <del> and <ins>, but stripping attributes before passing them into htmltreediff can lead to confusing results.

Mainly, list types. To a lesser extent, alignment, sizing, etc.

[caffodian]

Hmm. Maybe this makes more sense to have a 3rd type of "modified" marker for a node. Rather than simply inserted/deleted.