Allow changing the label markup

[zerolab]

While tbxforms marks optional labels with "(required)", we had requests to stick to the asterisk denoting required fields.

At the moment that is not possible unless one overrides the whole field.html or use JS.

Can we:

• add a required class to all labels for required fields (or optional fields marked as required) e.g. tbxforms-label--required
• allow passing additional classes to labels, or the wrapping container (I guess that is covered by I would like to be able to add my own class to a label #26. One can pass label_class to the Field definition, h/t @siimonevans). Although in our case it can be tedious to have to specify each field
• make the label and include that is easier to override in isolation?
• optionally have TBXFORMS_REQUIRED_LABEL_MARKER settings, defaulting to an empty string?

[siimonevans]

Yes please! Without this I've found I've had to write gnarly CSS (and/or JS) which feels unnecessary.

[kbayliss]

Solved by #78