Add UPnP support for Lighthouse (#1587)

Adding UPnP support will help grow the DHT by allowing NAT traversal for peers with UPnP supported routers.

## Issue Addressed

#927 

## Proposed Changes

Using IGD library: https://docs.rs/igd/0.10.0/igd/

Adding the  the libp2p tcp port and discovery udp port. If this fails it simply logs the attempt and moves on

## Additional Info



Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/src/cli.rs

@@ -75,6 +75,12 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("One or more comma-delimited base64-encoded ENR's to bootstrap the p2p network. Multiaddr is also supported.")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("disable-upnp")
+                .long("disable-upnp")
+                .help("Disables UPnP support. Setting this will prevent Lighthouse from attempting to automatically establish external port mappings.")
+                .takes_value(false),
+        )
         .arg(
             Arg::with_name("enr-udp-port")
                 .long("enr-udp-port")