[proposal] Add support for throttling per route

API Gateway has various forms of API throttling. This proposal adds support for throttling at the routing level. This allows you to specify a rate and burst limit per `@app.route`.

Similar to CORS and authorizers, there will be a new param, `throttle`, that takes an instance of `ThrottleConfig`:

```python

@app.route('/', throttle=ThrottleConfig(rate=10, burst=15))
def index(): pass
```

`ThrottleConfig` accepts two arguments, `rate` and `burst`:

```python
class ThrottleConfig(object):
def __init__(self, rate: int, burst: int): pass
```

The `rate` and `burst` params map to the corresponding concepts described in the [API Gateway throttling docs](http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-request-throttling.html)

Start at the @app.route implementation and inspect how the existing CORS and authorizer parameters are handled, then review the linked API Gateway throttling documentation. Done means routes can accept a ThrottleConfig with rate and burst values and those limits are mapped to the corresponding API Gateway concepts.

Written by the indexing model from the issue text.