Add graffiti cli flag to the validator client. (#1425)

## Issue Addressed

#1419

## Proposed Changes

Creates a `--graffiti` cli flag in the validator client. If the flag is set, it overrides graffiti in the beacon node. 

## Additional Info

validator_client/src/cli.rs

@@ -60,4 +60,12 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                       node is not synced.",
                 ),
         )
+        // This overwrites the graffiti configured in the beacon node.
+        .arg(
+            Arg::with_name("graffiti")
+                .long("graffiti")
+                .help("Specify your custom graffiti to be included in blocks.")
+                .value_name("GRAFFITI")
+                .takes_value(true)
+        )
 }