Add mode enum to control output of color sinks

This adds a new "color_mode" enum that can be used to control the color
code output behavior of sinks with color support. It can be one of three
values: always, automatic and never.

include/spdlog/common.h

@@ -159,6 +159,16 @@ inline spdlog::level::level_enum from_str(const std::string &name) SPDLOG_NOEXCE
 using level_hasher = std::hash<int>;
 } // namespace level
 
+//
+// Color mode used by sinks with color support.
+//
+enum class color_mode
+{
+    always,
+    automatic,
+    never
+};
+
 //
 // Pattern time - specific time getting to use for pattern_formatter.
 // local time by default