Alter context before creating project

It would be nice if cookiecutter could run a function into it's context before creating the project. Example:

```
project
-- {{ cookiecutter.repo_name }}/ ...
-- context.py
```

**context.py**

```
def update_context(context):
# do stuff to context
```

Start by tracing Cookiecutter's project-creation flow and how the template context is assembled; the issue's example uses a template-level context.py with update_context(context). Determine the integration point and API needed to modify context before rendering, then verify that a template function can affect the generated project context before creation.

Written by the indexing model from the issue text.