Added Default logic to refs

Not sure about the switch statement. There may be a more elegant solution.

Part of #2484

License: MIT
Signed-off-by: Richard Littauer <richard.littauer@gmail.com>

core/commands/refs.go

@@ -50,7 +50,7 @@ NOTE: List all references recursively by using the flag '-r'.
 		cmds.StringArg("ipfs-path", true, true, "Path to the object(s) to list refs from.").EnableStdin(),
 	},
 	Options: []cmds.Option{
-		cmds.StringOption("format", "Emit edges with given format. Available tokens: <src> <dst> <linkname>."),
+		cmds.StringOption("format", "Emit edges with given format. Available tokens: <src> <dst> <linkname>.").Default("<dst>"),
 		cmds.BoolOption("edges", "e", "Emit edge format: `<from> -> <to>`.").Default(false),
 		cmds.BoolOption("unique", "u", "Omit duplicate refs from output.").Default(false),
 		cmds.BoolOption("recursive", "r", "Recursively list links of child nodes.").Default(false),