`border` and `line` options should allow catalogue indexes

[Axlefublr]

I really appreciate you designing the options in a way where they can use whatever characters the user want, that's very good design.

However, I like the default arrangement of borders in the :CBcatalogue (while hating the 1st, default one, specifically).
So what I'm led to do is go into the catalogue, copy all the symbols of the border, and paste them back into my configuration.

That's not a big deal really, but I think it would make sense to allow the user to just pass a number to the two options, to mean one of the indexes in the catalogue. I think it will be convenient.
I'm interested in making a PR implementing this, if you agree with the idea and once you're ready to take in new PRs.