Allow value for beacon_node fee-recipient argument (#2884)

## Issue Addressed

The fee-recipient argument of the beacon node does not allow a value to be specified:

> $ lighthouse beacon_node --merge --fee-recipient "0x332E43696A505EF45b9319973785F837ce5267b9"
> error: Found argument '0x332E43696A505EF45b9319973785F837ce5267b9' which wasn't expected, or isn't valid in this context
> 
> USAGE:
>    lighthouse beacon_node --fee-recipient --merge
>
> For more information try --help

## Proposed Changes

Allow specifying a value for the fee-recipient argument in beacon_node/src/cli.rs

## Additional Info

I've added .takes_value(true) and successfully proposed a block in the kintsugi testnet with my own fee-recipient address instead of the hardcoded default. I think that was just missed as the argument does not make sense without a value :)


Co-authored-by: pk910 <philipp@pk910.de>
Co-authored-by: Michael Sproul <micsproul@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/src/cli.rs

@@ -402,11 +402,13 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("fee-recipient")
                 .long("fee-recipient")
+                .value_name("FEE-RECIPIENT")
                 .help("Once the merge has happened, this address will receive transaction fees \
                        collected from any blocks produced by this node. Defaults to a junk \
                        address whilst the merge is in development stages. THE DEFAULT VALUE \
                        WILL BE REMOVED BEFORE THE MERGE ENTERS PRODUCTION")
                 .requires("merge")
+                .takes_value(true)
         )
 
         /*