add quickStart option that avoids network metadata

beacon_chain/beacon_node.nim

@@ -164,6 +164,7 @@ proc init*(T: type BeaconNode, conf: BeaconNodeConf): Future[BeaconNode] {.async
     stderr.write args, "\n"
     quit 1
 
+  if not conf.quickStart:
     case conf.network
     of "mainnet":
       fail "The Serenity mainnet hasn't been launched yet"

beacon_chain/conf.nim

@@ -31,6 +31,10 @@ type
       shortform: "n"
       defaultValue: DEFAULT_NETWORK .}: string
 
+    quickStart* {.
+      desc: "Run in quickstart mode",
+      defaultValue: false.}: bool
+
     dataDir* {.
       desc: "The directory where nimbus will store all blockchain data."
       shortform: "d"