This commit allows quoting of discourse-local-date elements
and converts the quoted tags back into bbcode so that the
rendered quote will also render the discourse-local-date HTML.
This works on single dates as well as date ranges, and supports
all of the options used by discourse-local-date.

This also necessitated adding addTextDecorateCallback to the
to-markdown core lib (similar to addBlockDecorateCallback and
addTagDecorateCallback) to transform the text nodes between
date ranges to remove the -> in the final quote.

c.f. https://meta.discourse.org/t/quotes-that-contain-date-time/101999