Allow configuring delimiter for date / datetimes

[chrisdmacrae]

I'd love to be able to configure the delimiters the plugin uses to find dates / datetimes in each list, as I use the "tasks" plugin, and it looks for dates in this format at the end of a list:

- [ ] My task or card 📅 2021-01-01 00:00

Whereas the kanban app does:

- [ ] My task or card @{2021-01-01 00:00}

I'd be willing to submit a PR for this change once a contributor weighs in.

[schemar]

Thanks @chrisdmacrae,

I think this is a duplicate of #69 where you would add one more format, is that right?

[chrisdmacrae]

@schemar you're absolutely right