Fix #59: add_input_classes as filter

[codeout]

add_input_classes should return field to use it as a filter.

--- a/bootstrapform/templatetags/bootstrap.py
+++ b/bootstrapform/templatetags/bootstrap.py
@@ -45,20 +45,21 @@ def bootstrap_horizontal(element, label_cols='col-sm-2 col-lg-2'):

     return render(element, markup_classes)

 @register.filter
 def add_input_classes(field):
     if not is_checkbox(field) and not is_multiple_checkbox(field) \
        and not is_radio(field) and not is_file(field):
         field_classes = field.field.widget.attrs.get('class', '')
         field_classes += ' form-control'
         field.field.widget.attrs['class'] = field_classes
+    return field

I would open one-line PR if you want. Thanks.