allow callback to modify required contents

Use case:
if you do things like this:
       // (coffeescript) a example config file in config folder /path/to/configs/my.coffee
       module.exports = (opts)->
          name: "chris"
          environment: if opts.env is “dev” then “development” else “production"


        // app code
        // goes through every file in the config folder and if it is a function executes it, and returns that instead of the contents.
        var opts = {dev: true};
        configuration = requireDirectory(module,”/path/to/configs",null,function(err,mod){
            return (typeof mod === "function") ? mod(opts) : mod;
        });

        // results in
        configuration.my == {
            name: ‘chris’,
            environment: ‘development'
        };