Use Django System Check Framework to check validity of defined tables

[waseem-omar]

Hi all, thank you for building such an awesome library!

I have a suggestion to use Django System Check Framework to check if table classes and their columns are defined properly before rendering the table.

For example, Column.empty_values attribute is expected to be a list or tuple, and can become problematic in some cases if it is set as a string. With System Check, such issue can be caught upon application startup instead of facing an error upon rendering the table.

This can become a big change and may be of a low priority, however, we can start by registering a checker for django_tables2 that checks for empty_values attribute type, and add to it incrementally.