add label attrs to BaseForm

[Christoph]

Goal:
Add attributes like "class" to the label tag for inputs.

Problem:
Actually the only thing thats stops me from using {{ form }} is the incapability to change classes for labels easily. So i have to do it like this, for every form and every field:

{{ form.non_field_errors }}
<div class="fieldWrapper">
    {{ form.subject.errors }}
    <label for="{{ form.subject.id_for_label }}" class="MY-CUSTOM-CLASS">Email subject:</label>
    {{ form.subject }}
</div>

Where it should be changed:
I would be fine if the label_attrs could be set in BaseForm(), but i think the right place would be class Field

As the label_tag is already set up to give the label class attributes and label_suffix, it is quite easy to add this to the BaseForm _html_output()

class BoundField:
    def label_tag(self, contents=None, attrs=None, label_suffix=None):

class BaseForm:
    def _html_output(self, normal_row, error_row, row_ender, help_text_html, errors_on_separate_row):
        # ...
        label = bf.label_tag(label) or ''

[Tim Graham]

I don't understand what change you're proposing. Can you provide a patch or describe the changes and the use case?