Aceept also 'warn' string in level::from_str()

include/spdlog/common-inl.h

@@ -34,7 +34,8 @@ SPDLOG_INLINE spdlog::level::level_enum from_str(const std::string &name) SPDLOG
         }
         level++;
     }
-    return level::off;
+    // allow warn = warning before giving up
+    return name == "warn" ? level::warn: level::off;
 }
 } // namespace level