add feature toggles example

[jlhood]

It'd be great to add an example of supporting feature toggles in this example app. Approach:

1. Reuse SSM for feature toggles. Can use dedicated namespace like /applications/apprepo/${Stage}/features/....
2. Should support per-customer feature enable like we do in prod.
3. ApiLambda code can have an if check for some example feature.
4. Add ops tool Lambda function to ops stack for flipping feature toggles to demo our approach to ops tools written as Lambda functions. Function can include logging auditing information.

When adding this, we should also add a wiki page for feature toggles in Patterns and Best Practices.