Map cognito user pool claims as part of context

[jamesls]

The docs show how to integrate cognito user pools with API gateway, and include an example of how to map through the various claims:

{
	"context" : {
		"sub" : "$context.authorizer.claims.sub",
		"email" : "$context.authorizer.claims.email"
	}
}
...

This isn't included as part of the "method request passthrough" template that chalice uses, and probably should be.