Add feature flag for writing ssz pre/post to disk (#1046)

beacon_node/Cargo.toml

@@ -11,6 +11,9 @@ path = "src/lib.rs"
 [dev-dependencies]
 node_test_rig = { path = "../tests/node_test_rig" }
 
+[features]
+write_ssz_files = ["beacon_chain/write_ssz_files"]  # Writes debugging .ssz files to /tmp during block processing.
+
 [dependencies]
 eth2_config = { path = "../eth2/utils/eth2_config" }
 beacon_chain = { path = "beacon_chain" }