Add append mode. (#1847)

include/fmt/os.h

@@ -278,7 +278,8 @@ class file {
     RDONLY = FMT_POSIX(O_RDONLY),  // Open for reading only.
     WRONLY = FMT_POSIX(O_WRONLY),  // Open for writing only.
     RDWR = FMT_POSIX(O_RDWR),      // Open for reading and writing.
-    CREATE = FMT_POSIX(O_CREAT)    // Create if the file doesn't exist.
+    CREATE = FMT_POSIX(O_CREAT),   // Create if the file doesn't exist.
+    APPEND = FMT_POSIX(O_APPEND)   // Open in append mode.
   };
 
   // Constructs a file object which doesn't represent any file.