Add the ability to swap the underlying exporter

[valentinogagliardi]

First of all, thanks for this fantastic library! I'm using it for a bunch of tables in a Django project, and I really love the simplicity and ease of use.

For my use case, I had to customize things a bit to get custom styling in the exported Excel, and I don't like how I did it.

From what I understand, there is currently no clean method to swap the underlying data structure, and its exporter, used in https://github.com/jieter/django-tables2/blob/master/django_tables2/export/export.py#L65

What I ended up doing is a subclass of TableExport, where I overrode the Dataset class.

Another thing I don't like of tablib is that there is no way as far as I can tell to customize the styles in XLSXFormat.

It is something feasible?

With the proper guidance, I'd love to contribute with a pull request.

Thanks!