Minor extension: allow apps to override the type label in the add form easily

When overriding apps, this turns out to be a very useful feature to
have.

polymorphic/admin.py

@@ -25,7 +25,15 @@ class PolymorphicModelChoiceForm(forms.Form):
     """
     The default form for the ``add_type_form``. Can be overwritten and replaced.
     """
-    ct_id = forms.ChoiceField(label=_("Type"), widget=AdminRadioSelect(attrs={'class': 'radiolist'}))
+    #: Define the label for the radiofield
+    type_label = _("Type")
+
+    ct_id = forms.ChoiceField(label=type_label, widget=AdminRadioSelect(attrs={'class': 'radiolist'}))
+
+    def __init__(self, *args, **kwargs):
+        # Allow to easily redefine the label (a commonly expected usecase)
+        super(PolymorphicModelChoiceForm, self).__init__(*args, **kwargs)
+        self.fields['ct_id'].label = self.type_label