interp: Support more flags on the "read" builtin

Originally asked on https://github.com/go-task/task/issues/314.

There are some missing options on the read builtin. The -p one seems particularly useful on scripts.

https://github.com/mvdan/sh/blob/b7aede07e8af0320deb8b5f835bb1df6ec59face/interp/builtin.go#L473-L479