interp: Support more flags on the "read" builtin

[andreynering]

Originally asked on go-task/task#314.

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

sh/interp/builtin.go

Lines 473 to 479 in b7aede0

	switch args[0] {
	case "-r":
	raw = true
	default:
	r.errf("read: invalid option %q\n", args[0])
	return 2
	}

[mvdan]

Sure - patches welcome :)

[blackjid]

I think is already supported.

I wonder if the read -s silent would be possible to be able to read password or secrets.