Prepare for termbox/windows build

`TAGS=termbox make` (or `go build -tags termbox`)

src/tui/tui_test.go

@@ -0,0 +1,14 @@
+package tui
+
+import (
+	"testing"
+)
+
+func TestPairFor(t *testing.T) {
+	if PairFor(30, 50) != PairFor(30, 50) {
+		t.Fail()
+	}
+	if PairFor(-1, 10) != PairFor(-1, 10) {
+		t.Fail()
+	}
+}