Introduce `app` object and routing

[tomas-polach]

Currently, there is no app object and the use method is called directly:

const { use } = require('lambda-expressless');
const bodyParser = require('body-parser');

exports.handler = use(bodyParser.json(), (req, res) => {
  res.status(201).json({
    success: false,
    data: req.body.id
  })
});

Proposal: Add a workflow using the app object, compatible with Express way of handling middleware and routing:

const Expressless = require('lambda-expressless');
const bodyParser = require('body-parser');

const app = Expressless()

app.use(bodyParser.json())
app.use('/do-it', authenticate, doIt)
app.use('/do-other-stuff', authenticate, doOtherStuff)
app.use(handleErrors)

exports.handler = app.apiGatewayHandler