Add NOTICE, ALERT and EMERG log level.

This commit introduces 3 new log level. Thoses are:
    + NOTICE, which is a bit worse that INFO, but still not a warn.
    + ALERT, for case worse that critical.
    + EMERG, application is unusable.

With those 3 log levels, spdlog now has all log level accepted by
the syslog() system call.

include/spdlog/common.h

@@ -49,14 +49,18 @@ typedef enum
     TRACE,
     DEBUG,
     INFO,
+    NOTICE,
     WARN,
     ERR,
     CRITICAL,
+    ALERT,
+    EMERG,
     ALWAYS,
     OFF
 } level_enum;
 
-static const char* level_names[] { "trace", "debug", "info", "warning", "error", "critical", "", ""};
+static const char* level_names[] { "trace", "debug", "info", "notice", "warning", "error", "critical",
+        "alert", "emerg", "", ""};
 inline const char* to_str(spdlog::level::level_enum l)
 {
     return level_names[l];