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];

include/spdlog/details/logger_impl.h

@@ -101,6 +101,12 @@ inline spdlog::details::line_logger spdlog::logger::info(const Args&... args)
     return log(level::INFO, args...);
 }
 
+template <typename... Args>
+inline spdlog::details::line_logger spdlog::logger::notice(const Args&... args)
+{
+    return log(level::NOTICE, args...);
+}
+
 template <typename... Args>
 inline spdlog::details::line_logger spdlog::logger::warn(const Args&... args)
 {
@@ -119,6 +125,18 @@ inline spdlog::details::line_logger spdlog::logger::critical(const Args&... args
     return log(level::CRITICAL, args...);
 }
 
+template <typename... Args>
+inline spdlog::details::line_logger spdlog::logger::alert(const Args&... args)
+{
+    return log(level::ALERT, args...);
+}
+
+template <typename... Args>
+inline spdlog::details::line_logger spdlog::logger::emerg(const Args&... args)
+{
+    return log(level::EMERG, args...);
+}
+
 inline const std::string& spdlog::logger::name() const
 {
     return _name;

include/spdlog/logger.h

@@ -75,9 +75,12 @@ public:
     template <typename... Args> details::line_logger trace(const Args&... args);
     template <typename... Args> details::line_logger debug(const Args&... args);
     template <typename... Args> details::line_logger info(const Args&... args);
+    template <typename... Args> details::line_logger notice(const Args&... args);
     template <typename... Args> details::line_logger warn(const Args&... args);
     template <typename... Args> details::line_logger error(const Args&... args);
     template <typename... Args> details::line_logger critical(const Args&... args);
+    template <typename... Args> details::line_logger alert(const Args&... args);
+    template <typename... Args> details::line_logger emerg(const Args&... args);
 
 
 private: