Add a method to set attributes on tbody

[Fran-Rg]

The current tbody element is created without element passed to it: https://github.com/plumdog/flask_table/blob/master/flask_table/table.py#L134

It can be practical to add a tbody_attrs = {"class": "text-xs"} class

[plumdog]

Yep, I reckon something like tbody_html_attrs would fit in with https://github.com/plumdog/flask_table#table-configuration-and-options

Are you able to open a PR?