Allow customisation of header names when rendering a collection

[michiels]

The CsvExport#render_collection(a_collection) method currently uses the fields passed into CsvExport.new to read column values from the objects in a_collection. This makes it really easy to provide clear-cut CSV exports.

However, sometimes we want to name columns just a little bit different than their underlying field name or virtual attribute.

For example, we could update the #new method to support the following:

export = CsvExport.new(id: :number, name: :full_name)
export.render_collection(@customers)