[wip] Expose watching functionality via npm

[emilymcafee]

Beginning to mold conex watching functionality to follow stork's example. Started by rewriting watch.sh in javascript.

const conex = require('@mapbox/ecs-conex').watch;

const options = {
  region: 'us-east-1',
  suffix: 'production',
  token: 'xxx',
  org: 'mapbox',
  repo: 'repo'
};

conex(options)
  .then(() => {
    console.log('mapbox/repo being watched by ecs-conex-production in us-east-1');
  });

Todo:

• [ ] Decide if we want to write a "watch" lambda function, that a user can run with encrypted Github credentials saved in the environment instead of needing to store those credentials locally.
• [ ] Tests \o/
• [ ] Adjust linting + test scripts to catch correct files.
• [ ] Decide if we want to deprecate bash functionality that does the same.
• [ ] Docs, changelog, etc
• [ ] Publish an npm module so that a user invoke the watch script (lambda or not) in javascript.

Would love a gutcheck from @rclark before I head down this path :point_up: