Ability to specify custom Vercel environment

[asennikov]

Same issue as here: BetaHuhn#396

Vercel now supports custom environments beyond Dev, Preview and Production, see https://vercel.com/docs/cli/deploy#target

There's a PR proposed in the originally forked repo, would you consider merging it here too?

BetaHuhn#397

[asennikov]

Sorry, meant to open an issue, here it is: #26