Add -tor switch that uses custom tor.

For reasons outlined in #2, we want to be
able to use a custom tor for now.  This patch introduces a new command
line switch, -tor, which makes this possible.

main.go

@@ -119,6 +119,7 @@ func main() {
 	var certFilename, keyFilename string
 	var cacheFile string
 	var templatesDir string
+	var torBinary string
 	var numSecs int
 
 	flag.StringVar(&addr, "addr", ":5000", "Address to listen on.")
@@ -130,6 +131,7 @@ func main() {
 	flag.StringVar(&keyFilename, "key", "", "TLS private key file.")
 	flag.StringVar(&cacheFile, "cache", "bridgestrap-cache.bin", "Cache file that contains test results.")
 	flag.StringVar(&templatesDir, "templates", "templates", "Path to directory that contains our web templates.")
+	flag.StringVar(&torBinary, "tor", "tor", "Path to tor executable.")
 	flag.IntVar(&numSecs, "seconds", 0, "Number of seconds after two subsequent requests are handled.")
 	flag.Parse()
 
@@ -165,7 +167,7 @@ func main() {
 		return
 	}
 
-	torCtx = &TorContext{}
+	torCtx = &TorContext{TorBinary: torBinary}
 	if err = torCtx.Start(); err != nil {
 		log.Printf("Failed to start Tor process: %s", err)
 		return

tor.go

@@ -105,6 +105,7 @@ type TorContext struct {
 	DataDir   string
 	Cancel    context.CancelFunc
 	Context   context.Context
+	TorBinary string
 	eventChan chan *bulb.Response
 }
 
@@ -158,7 +159,7 @@ func (c *TorContext) Start() error {
 
 	// Start our Tor process.
 	c.Context, c.Cancel = context.WithCancel(context.Background())
-	cmd := exec.CommandContext(c.Context, "tor", "-f", tmpFh.Name())
+	cmd := exec.CommandContext(c.Context, c.TorBinary, "-f", tmpFh.Name())
 	if err = cmd.Start(); err != nil {
 		return err
 	}