Custom widget attributes possible now, with prefix of widget_property__

Related to https://github.com/Yaco-Sistemas/django-inplaceedit/issues/56

I created the possibility to add attributes to the widgets themselves.
Not 100% sure if this is the most clean implementation though...

You need to pass in your attribute with prefix 'widget_property__' in the widget.
E.g. if you have a textarea widget that you want to limit in length and rows:

{% inplace_edit "custom_field" widget_property__maxlength=140, widget_property__rows=4 %}