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.

lighthouse/src/main.rs

@@ -20,7 +20,6 @@ pub const VERSION: &str = git_version!(
     fallback = crate_version!()
 );
 pub const DEFAULT_DATA_DIR: &str = ".lighthouse";
-pub const CLIENT_CONFIG_FILENAME: &str = "beacon-node.toml";
 pub const ETH2_CONFIG_FILENAME: &str = "eth2-spec.toml";
 
 fn main() {