Add eviction handler

[luzlab]

Added a method on to specify a callback that gets called when a key is expired.

[coveralls]

Coverage remained the same at 100.0% when pulling 90fad51 on luzlab:master into 9a40d7c on DylanPiercey:master.

[milkcha]

@DylanPiercey
i think this is our last requirement for this awesome module

[luzlab]

@DylanPiercey @milkcha I'm not certain about is whether we should call the handler before or after the item is evicted. Any thoughts?

[DylanPiercey]

@luzlab the main concern I have with this is that we are creating an event listener instance for a single event. It seems a bit wasteful to me.

If it suites your use-case I think it'd be better just to have something like:

const cache = new Receptacle();

cache.onDelete = (key, value) => {
  console.log("deleted", key, "which was", value);
}

Or just having it as an option to receptacle, eg:

new Receptacle({
  onDelete(k, v){ /* ... */ }
})

Thoughts?

[milkcha]

i would like

new Receptacle({
  onDelete(k, v){ /* ... */ }
})

it keep the lib simple and excellent quick. no need an event system. if we need , we can dispatch then event to other event lib use onDelete.
and
I'm not certain about is whether we should call the handler before or after the item is evicted too.