Support to not displaying labels for form inputs

When a WTForms field has no label, a label is generated from the variable name it is assigned to, e.g. for a `TextAreaWidget`. This is very useful (convention over configuration).

However, in certain [cases](https://getbootstrap.com/docs/4.4/components/forms/#without-labels), the label of an input is not added to thre resulting HTML. What parameter can be used when creating a WTForm field in order to extend the form template here to not add the label around an input?

This relates to:
- https://github.com/greyli/bootstrap-flask/blob/master/flask_bootstrap/templates/bootstrap/form.html#L47
- https://github.com/greyli/bootstrap-flask/blob/master/flask_bootstrap/templates/bootstrap/form.html#L70
- https://github.com/greyli/bootstrap-flask/blob/master/flask_bootstrap/templates/bootstrap/form.html#L124
- https://github.com/greyli/bootstrap-flask/blob/master/flask_bootstrap/templates/bootstrap/form.html#L152