Prepare for termbox/windows build

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

src/util/util_unix.go

@@ -0,0 +1,17 @@
+// +build !windows
+
+package util
+
+import (
+	"os"
+	"os/exec"
+)
+
+// ExecCommand executes the given command with $SHELL
+func ExecCommand(command string) *exec.Cmd {
+	shell := os.Getenv("SHELL")
+	if len(shell) == 0 {
+		shell = "sh"
+	}
+	return exec.Command(shell, "-c", command)
+}