Ability to provide additional event emitter to emit on

[hgrubst]

We call migrate on the startup of our application for each service that composes the application.
Whilst we can definitely use the migrate.emitter to log what is happening in our application, the fact that it is a shared instance on the migrate object makes it hard to log on the appropriate log category.
Here is a code example of the issue we face :

//service 1
log = getLogger('service1')
migrate.emitter.on('info', (data)=> log.debug(data))
migrate(...)

//service 2
log = getLogger('service2')
migrate.emitter.on('info', (data)=> log.debug(data))
//this is now registering a second listener to the same emitter which will end up logging stuff from service 1 as well
migrate(...)

I can think of other was to solve this, eg making migrate a class that you instanciate intead of shared instance so that each instance would have its own emitter.
Or you could also have a 'label' option passed in to migrate which could be used to emit more specifically :

//instead of 
internals.emitter.emit(name, data)
//do
internals.emitter.emit(`${name}-${options.label}`, data)

[coveralls]

Coverage decreased (-0.2%) to 97.079% when pulling 85191a7 on hgrubst:emitter-option into fae1f6d on vinicius0026:master.