Enhancement: Accept a custom normalizer if type is a function

[cheton]

I suggest that the second parameter type should also accept a function as custom normalizer.

For example:

var request = require('superagent');
var serializer = require('superagent-serializer');

var customNormalizer = function(serializedRes) {
    // custom processing
    return serializedRes;
};

serializer(request, customNormalizer);

[zzarcon]

This is a great idea!

Happy to get merge it if you have time for PR :)