Add privacy option (#2016)

Adds a `--privacy` CLI flag to the beacon node that users may opt into. 

This does two things:
- Removes client identifying information from the identify libp2p protocol
- Changes the default graffiti to "" if no graffiti is set.

beacon_node/src/cli.rs

@@ -97,6 +97,12 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .help("Disables UPnP support. Setting this will prevent Lighthouse from attempting to automatically establish external port mappings.")
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("private")
+                .long("private")
+                .help("Prevents sending various client identification information.")
+                .takes_value(false),
+        )
         .arg(
             Arg::with_name("enr-udp-port")
                 .long("enr-udp-port")