Added an optional local port definition (#4)

This commit simply adds the support for a local port definition when creating a tunnel.

README.md

@@ -29,6 +29,10 @@ tunnel := sshtunnel.NewSSHTunnel(
 
    // The destination host and port of the actual server.
    "dqrsdfdssdfx.us-east-1.redshift.amazonaws.com:5439",
+
+   // The local port you want to bind the remote port to.
+   // Specifying "0" will lead to a random port.
+   "8443",
 )
 
 // You can provide a logger for debugging, or remove this line to

ssh_tunnel.go

@@ -83,9 +83,10 @@ func (tunnel *SSHTunnel) Close() {
 	return
 }
 
-func NewSSHTunnel(tunnel string, auth ssh.AuthMethod, destination string) *SSHTunnel {
-	// A random port will be chosen for us.
-	localEndpoint := NewEndpoint("localhost:0")
+// NewSSHTunnel creates a new single-use tunnel. Supplying "0" for localport will use a random port.
+func NewSSHTunnel(tunnel string, auth ssh.AuthMethod, destination string, localport string) *SSHTunnel {
+
+	localEndpoint := NewEndpoint("localhost:"+localport)
 
 	server := NewEndpoint(tunnel)
 	if server.Port == 0 {