Reduce boilerplate for single stack apps

[eladb]

It is a very common case to have CDK apps with a single stack, in which case there's currently too much boilerplate:

import { App, Stack } from 'aws-cdk'; // boilerplate
import { Topic } from '@aws-cdk/sns';
import { Table } from '@aws-cdk/dynamodb';

// boilerplate
const app = new App(process.argv);
const stack = new Stack(app, 'my-stack');

// actual interesting code
new Topic(stack, 'MyTopic');
new Table(stack, 'MyTable');

// boilerplate
process.stdout.write(app.run());

The vision is something like:

import { stack } from 'aws-cdk'; // wish we could get rid of this too, but that's ok
import { Topic } from '@aws-cdk/sns';
import { Table } from '@aws-cdk/dynamodb';

new Topic(stack, 'MyTopic');
new Table(stack, 'MyTable');

Then, don't even need to run tsc:

cdk -a my-app.ts --name my-stack synth
Resources:
  MyTopic:
    Type: AWS::SNS::Topic
  MyTable:
    Type: AWS::DynamoDB::Table
    # ...

Naturally, we need a solution that works for all supported languages.

[eladb]

Dup #216