Allow computable attrs for <td> from table.attrs

[jieter]

It was already possible to use the attrs-argument like this:

class Table(tables.Table):
    class Meta:
        attrs = {
           'td': {'class': 'foo'}
        }

But it did not pass any relevant arguments to callables. This PR changes that and passes column and table keyword arguments if appropriate:

class Table(tables.Table):
    class Meta:
        attrs = {
           'td': {'data-name': lambda column: column.name}
        }

Fixes #451