correct field_choices.html to make it compilable
this patch add the support to django-templates_preprocessor
compiler to remove useless spaces in output of bootstrap forms
i just replaced the line
```
{% for attr,value in field.field.widget.attrs.items %}{{ attr }}="{{
value }}" {% endfor %}
```
by a {{ field.field.widget.attrs|display_attr}} with the newly created
tag `display_attr`
corrected the missing tag exception
forgoten to add a {% load bootstrap_toolkit %}
Merge pull request #63 from onysos/master

make templates compatible with django-template-preprocessor