Allow overriding the default generation of rest arguments in js-es <= 5

[R32]

This gives the option to use the code generated by the compiler or not.

function rest( name : String, args : haxe.Rest<String> ) {
#if (js_es < 6)
	var args = js.Syntax.code("arguments"); // <-- overrides
	var i = 1;
#else
	var i = 0;
#end
    while(i < args.length)
        trace(name + " : " + args[i++]);
}

generated es5:

function rest(name) {
	var args = arguments;
	var i = 1;	
	while(i < args.length) console.log(name + " : " + args[i++]);
}