Added support for `clear` command on macOS (darwin)

This fixes the issue "Error: your platform is unsupported, terminal can't be cleared" when trying to convert a GIF in macOS.

aic_package/util.go

@@ -127,6 +127,7 @@ func init() {
 		cmd.Stdout = os.Stdout
 		cmd.Run()
 	}
+	clear["darwin"] = clear["linux"]
 }
 
 func clearScreen() {