Add to vercel.json, rather than overwriting it.

[mroswell]

I'd like to be able to add to vercel.json. But Datasette overwrites whatever I put in that file. I originally reported this here:
simonw/datasette-publish-vercel#51

In that case, I wanted to do a rewrite... and now I need to do 301 redirects (because we had to rename our site).

Can this be addressed?

[simonw]

Addressing that over here (hadn't seen that issue yet, thanks for the prod): simonw/datasette-publish-vercel#51 (comment)

[simonw]

Fixed in the 0.12 release of that plugin: https://github.com/simonw/datasette-publish-vercel/releases/tag/0.12