feat(core/processManager): introduce `processes:stop` handlers
feat(core/processManager): introduce `processes:stop` handlers

So far, `ProcessManager` was able to only register a `process:launch` handler.
There was no way to tell `ProcessManager` how to stop processes. This hasn't
been a problem so far as most of the service processes can be started without the usage
of the `ProcessManager`, but turns out to be necessary if we want Embark UI to be able
to pick up running services.

A good example is the webserver process, which until now bypasses the `ProcessManager`
all together. The webserver sets up two event handlers to start and stop it respectively:

```
this.events.setCommandHandler('start-webserver', () => this.server.start());
this.events.setCommandHandler('stop-webserver', () => this.server.stop());
```

In the future, this should happen through the `ProcessManager` instead, so the webserver
process can be picked up by Embark UI, like this:

```
this.request('process:register', 'webserver', () => {
  this.server.start();
});

// and then

this.request('process:launch', 'webserver', () => {
  // server started
});
```

Notice that the given callback to registering a process is actually the function that
gets called to launch the process.

Having that in mind, and considering that we also need a way to stop the process through
`ProcessManager, so we don't introduce a regression, we need a way to register a stop
call back as well.

The new API introduced in this commit looks like this:

```
this.request('process:register', 'webserver', {
  launchFn: (callback) => { this.server.start(callback) },
  stopFn: (callback) => this.server.stop(callback) }
});

// and then

this.request('process:launch', 'webserver', (err, message, port) => {
  // server started
});

this.request('process:stop', 'webserver', err => {
  // server stopped
});
```

Notice that `process:register` works exactly the same way as before as well.

Another thing to notice is that all parameters emitted by the underlying process
are propagated to the outside caller, which is why `err`, `message` and `port` are
available inside the launch callback.

iurimatias/embark-framework,iurimatias/embark-framework