Allow custom toCSV function for exporting

[JedWatson]

We should add support for a custom .toCSV method to be specified on List schemas, and if present, use it to generate the data for export when csvs are downloaded in the Admin UI.

It should use dependency injection to allow (a) the current req or user to be passed in, and (b) a callback to be provided.

If a callback is required, then the data should be built up asynchronously - this would allow other queries to be run when generating the csv data.

Otherwise, the data should be returned.

[JedWatson]

Implemented.

An example of a custom, asynchronous toCSV method on Posts (from the yeoman generator) that populates categories and converts them to keys:

Post.schema.methods.toCSV = function(callback) {

    var post = this,
        rtn = this.toJSON();

    delete rtn.image;

    this.populate('categories', function() {
        rtn.categories = post.categories.map(function(i) { return i.key });
        callback(null, rtn);
    });

};

An example of a simple (non-async) toCSV method whose results are customised for the current user:

Post.schema.methods.toCSV = function(user) {

    return {
        id: post.id,
        name: post.name,
        yours: (user.id == this.author) ? true : false
    };

};

[bladey]

This is a really great feature, thanks Jed.