Allow to run in the background

As simple as this command is, this is truly required on any setup that requires testing.

Ideally in my mocha tests, I would write in the global_config.js:

```
/* eslint-disable */
const dynamodbLocal = require('dynamodb-localhost');

// setup the database
before(async function () {
await dynamodbLocal.install();
await dynamodbLocal.start({ port: 8000 });
});

// teardown
after(async function() {
await dynamodbLocal.stop();
});

// run before each test
beforeEach(() => {});

// run after each test
afterEach(() => {});
```

Obviously with other test files next to it, who just make a dynamodb connection to the local test database.
This setup, the serverless plugin, or the original dynamodb-local, none of them support the option to run this in the background, as such, these are all useless when attempting to run a CI/CD job on. I can't tell a pipeline to "press ctrl-c" when you're done.

Start by tracing the existing dynamodb-localhost install(), start(), and stop() entry points and the command used to launch the local database. Check how the Mocha global_config.js example invokes these lifecycle methods in CI/CD, then verify that the background process can be started and stopped without requiring an interactive Ctrl-C.

Written by the indexing model from the issue text.