Feature request: add "∴" to default shortcodes

[wbadart]

Hey team-

First off, loving argdown so far. I've started using it at work to help clarify arguments for technical decisions.

On to business: would it be possible to add the "therefore" unicode symbol ("∴") to the default shortcodes? I find myself using it a lot (especially in expanded pcs inferences) and it's a little cumbersome to add that custom shortcode to just about every argdown document I write. I totally understand any desire to keep that defaults list small, given the ability for users to extend it arbitrarily, but "therefore" is such a core concept in logic that this would be a great quality-of-life improvement for me (and hopefully others too).

For the actual shortcode, I've mainly been using :therefore:, but something like :.;: may fit better with the abbreviated style of the other logic symbols.

Thanks for your consideration!