Add an engine for global webrtc settings

* hook up UDP limitation to it.
This was Michiel De Backker's suggestion

relates to #281

settingengine.go

@@ -0,0 +1,28 @@
+package webrtc
+
+import "github.com/pions/webrtc/pkg/ice"
+
+var defaultSettingEngine = newSettingEngine()
+
+type settingEngine struct {
+	EphemeralUDP struct {
+		PortMin uint16
+		PortMax uint16
+	}
+}
+
+// SetEphemeralUDPPortRange limits the pool of ephemeral ports that
+// ICE UDP connections can allocate from
+func SetEphemeralUDPPortRange(portMin, portMax uint16) error {
+	if portMax < portMin {
+		return ice.ErrPort
+	}
+
+	defaultSettingEngine.EphemeralUDP.PortMin = portMin
+	defaultSettingEngine.EphemeralUDP.PortMax = portMax
+	return nil
+}
+
+func newSettingEngine() *settingEngine {
+	return new(settingEngine)
+}