Add log-format option to CLI (#744)

* Add log-format CLI option

* Cargo fmt

* Add log format logic for file logging. Add doc

* Review comment

* Fix compilation errors

* Remove Mutex from logger

lcli/src/main.rs

@@ -249,7 +249,7 @@ fn run<T: EthSpec>(env_builder: EnvironmentBuilder<T>, matches: &ArgMatches) {
     let env = env_builder
         .multi_threaded_tokio_runtime()
         .expect("should start tokio runtime")
-        .async_logger("trace")
+        .async_logger("trace", None)
         .expect("should start null logger")
         .build()
         .expect("should build env");