[proposal] Add support for a wheels dir

Right now there are two directories consulted when packaging your app:

* `chalicelib/` - This is for code *you* write that's part of your chalice app. It's a middle ground between having to put everything in your app.py file and putting all your business logic in an entirely separate python package
* `vendor/` - A directory that just blindly copies its contents into the deployment package. While intended for python code, it can be anything including static assets and .so files.

However, another common use case is where you have a package in which you have a `.whl` file that works correctly in lambda. A common use case might be some internal dependency that's not available on pypi but you've correctly built a wheel for on an ec2 instance or a docker container. In the simplest case you'd just have to unzip into the `vendor/` directory, but it's one step removed from what's actually produced in a build step (a `.whl` file).

### Proposal

What I'd propose is we add another directory, `wheels/`, that contains a list of `.whl` files you want added to your deployment package. As part of the packaging process we'll automatically handle the correct unpacking of the whl into the deployment package. You'd have something like:

```
├── app.py
├── app.pyc
├── requirements.txt
├── vendor
└── wheels
├── magic-package.whl
└── special-internal-package.whl
```

### Rationale

We've seen some users struggle with vendor/, and it's not obvious how to go from a wheel file to something that would work in `vendor/`. Additionally, it's an unnecessary step. The `bdist_wheel` produces a `.whl` file, it seems reasonable that chalice should just work directly with wheels.

I realize that the number of options for packaging your app might be getting to the point where it's too confusing, but I do think this solves a use case that isn't cleanly addressed elsewhere.

Let me know what you all think.

Start by tracing the packaging process that currently handles the vendor/ directory and deployment packages. Compare that flow with the proposed wheels/ directory and determine how .whl files should be unpacked. Done means wheel contents are included correctly in the deployment package, with coverage for the new packaging behavior.

Written by the indexing model from the issue text.