Event listening support

[mikesir87]

Would be nice to allow me to hook in functions to hear and act on various events (like newly exposed/removed ports). There's a few ways this could be done, so figured I'd start the discussion.

What I'd like to propose is API similar to this...

pwd.on('event-name', cbFunction) => Function - returned function removes the listener when invoked

Using this approach, I can add an arbitrary number of listeners at any point in time, making it easy to add new plugins, etc.

This issue is more about plugging in the event API and not adding a particular event hook yet, as I feel those should be separate issues and keeps this discussion more focuses. Thoughts?

Example

The parameters are ones I just picked and would be dependent on the actual event being published.

pwd.on('instance.ports.new', function(instanceMetaData, port, url) {
  // Add a button that will display the port and open the url when clicked
});

[mikesir87]

On second thought... maybe the .on method shouldn't return an unsubscribe function to allow for chaining of commands. There should be a separate .off (or similar) function that removes the event listener.

It looks like there are several event emitters already available too... making less code we'd have to maintain. Here's a library that already does event handling that we can simply integrate...

https://github.com/scottcorgan/tiny-emitter - super small and has a once method, which could be nice for one-time listeners (although not sure what they might be here)