Add ability to configure CORS header (#1345)

## Issue Addressed

https://github.com/sigp/lighthouse/issues/1177

## Proposed Changes

Add a command line option (`--http-allow-origin`) and a config item for configuring the `Access-Control-Allow-Origin` response header.  This should unblock making XMLHttpRequests.

beacon_node/src/cli.rs

@@ -161,6 +161,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .default_value("5052")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("http-allow-origin")
+                .long("http-allow-origin")
+                .value_name("ORIGIN")
+                .help("Set the value of the Access-Control-Allow-Origin response HTTP header.  Use * to allow any origin (not recommended in production)")
+                .default_value("")
+                .takes_value(true),
+        )
         /* Websocket related arguments */
         .arg(
             Arg::with_name("ws")