Add custom css classes to column definitions

Allows you to target columns with css classes for better styling, rather than outputting a column body with wasted markup.

example use, where you'd want numbers to be right-aligned in a table for better readability:

```python
class ExampleTable(tables.Table):
    name = column.Column()
    number_value = column.Column(css_class="ralign")

    table_sequence = ['name', 'number_value']
    table_empty = "No objects found."
```