Template tag "partial.py" renders template with sub-dict of context

When rendering a complex page using many partial templates, it's simpler to give the partials a subdict of the context rather than the entire context.

For example, if the context is:
```python
{'car': {'mileage': 9000, 'doors': 4},
 'house': {'sqft': 1800, 'baths': 2}}
```
and the template "myApp/partials/house.html" contains:
```html
<div>Area: {{ sqft }} sq. ft.</div>
<div>Bathrooms: {{ baths }}</div>
```
you could render it like this:
```html
{% load partial %}
{% partial myApp/partials/house.html house %}
```