Allow custom formatter in sink ctor

include/spdlog/sinks/sink.h

@@ -14,15 +14,10 @@ namespace sinks {
 class sink
 {
 public:
-    sink()
-        : level_(level::trace)
-        , formatter_(new pattern_formatter())
-    {
-    }
+    sink() = default;
 
-    explicit sink(std::unique_ptr<spdlog::pattern_formatter> formatter)
-        : level_(level::trace)
-        , formatter_(std::move(formatter))
+    explicit sink(std::unique_ptr<spdlog::formatter> formatter)
+        : formatter_{std::move(formatter)}
     {
     }
 
@@ -49,10 +44,10 @@ public:
 
 protected:
     // sink log level - default is all
-    level_t level_;
+    level_t level_{level::trace};
 
     // sink formatter - default is full format
-    std::unique_ptr<spdlog::formatter> formatter_;
+    std::unique_ptr<spdlog::formatter> formatter_{details::make_unique<spdlog::pattern_formatter>()};
 };
 
 } // namespace sinks