add emoji representation of log level

because we can't tweak Xcode 8 to use color in console
emoji is a viable workaround, and log implementaion could
use this.
robbiehanson/XcodeColors#88

src/lib.rs

@@ -299,6 +299,8 @@ const INITIALIZED: usize = 2;
 static MAX_LOG_LEVEL_FILTER: AtomicUsize = ATOMIC_USIZE_INIT;
 
 static LOG_LEVEL_NAMES: [&'static str; 6] = ["OFF", "ERROR", "WARN", "INFO", "DEBUG", "TRACE"];
+static LOG_LEVEL_EMOJIES: [&'static str; 6] = ["", "❤️", "💛", "💙",
+                                             "💚", "💜"];
 
 static SET_LOGGER_ERROR: &'static str = "attempted to set a logger after the logging system \
                      was already initialized";
@@ -435,6 +437,11 @@ impl Level {
         }
     }
 
+    /// Returns emoji representation of logging level
+    pub fn emoji(&self) -> &'static str {
+        LOG_LEVEL_EMOJIES[*self as usize]
+    }
+
     /// Returns the most verbose logging level.
     #[inline]
     pub fn max() -> Level {