MINOR: cli: add private pointer and release function

This pointer will be used for storing private context. With this,
the same executed function can handle more than one keyword. This
will be very useful for creation Lua cli bindings.

The release function is called when the command is terminated (give
back the hand to the prompt) or when the session is broken (timeout
or client closed).
diff --git a/include/proto/dumpstats.h b/include/proto/dumpstats.h
index db60bab..12f972c 100644
--- a/include/proto/dumpstats.h
+++ b/include/proto/dumpstats.h
@@ -413,8 +413,10 @@
 	const char *str_kw[5];   /* keywords ended by NULL, limited to 5
 				 separated keywords combination */
 	const char *usage;   /* usage message */
-	int (*parse)(char **args, struct appctx *appctx);
+	int (*parse)(char **args, struct appctx *appctx, void *private);
 	int (*io_handler)(struct appctx *appctx);
+	void (*io_release)(struct appctx *appctx);
+	void *private;
 };
 
 struct cli_kw_list {