Add custom classes

[simonrjones]

Is your feature request related to a problem? Please describe.
We'd like to add a custom class to this form code:

<div class="tbxforms-checkboxes">                                         

Describe the solution you'd like

Update this HTML to:

<div class="tbxforms-checkboxes tbxforms-checkboxes--small multicolumn">                                         

Options we've considered:

• Adding multicolumn as a label and targetting this in the template
• Adding HTML transformations to append a class where one is spotted

->appendClass('div.tbxforms-checkboxes', 'tbxforms-checkboxes--small multicolumn');

See https://symfony.com/doc/current/components/css_selector.html