add emoji to log level name for iOS

because we can't tweak Xcode 8 to use color in console
robbiehanson/XcodeColors#88

Author: king6cong

src/lib.rs

@@ -241,8 +241,12 @@ const INITIALIZED: usize = 2;
 
 static MAX_LOG_LEVEL_FILTER: AtomicUsize = ATOMIC_USIZE_INIT;
 
+#[cfg(not(target_os = "ios"))]
 static LOG_LEVEL_NAMES: [&'static str; 6] = ["OFF", "ERROR", "WARN", "INFO",
                                              "DEBUG", "TRACE"];
+#[cfg(target_os = "ios")]
+static LOG_LEVEL_NAMES: [&'static str; 6] = ["OFF", "❤️ ERROR", "💛 WARN", "💙 INFO",
+                                             "💚 DEBUG", "💜 TRACE"];
 
 /// An enum representing the available verbosity levels of the logging framework
 ///