all: add go.mod

Commands run:
	go mod init
	go mod edit -go=1.11
	go mod tidy
	go list -m all
	go test ./...

Updates golang/go#30228

Change-Id: I86819ac0a2704468903e34ca49308ce65149791b
Reviewed-on: https://go-review.googlesource.com/c/term/+/167160
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go.mod

@@ -0,0 +1,3 @@
+module golang.org/x/term
+
+go 1.11