Add SubscribingEventEmitter class

[glensc]

Problem: To register bulk of events, need to invoke ".on("eventname", handler) on each of the events, even handlers are coupled with the class itself, and class knows what events it wants to listen to.

Soliution: Add new class, which extends EventEmitter and adds addSubscriber method.

To make this appear for example to Worker class, it's parent must be updated:

diff --git a/src/core/worker.ts b/src/core/worker.ts
index 6d45c4b..761459a 100644
--- a/src/core/worker.ts
+++ b/src/core/worker.ts
@@ -6,6 +6,7 @@ import { WorkerOptions } from "../types/options";
 import { Connection } from "./connection";
 import { RunPlugins } from "./pluginRunner";
 import { Queue } from "./queue";
+import { SubscribingEventEmitter } from "../utils/SubscribingEventEmitter";

 function prepareJobs(jobs) {
   return Object.keys(jobs).reduce(function (h, k) {
@@ -100,7 +101,7 @@ export type WorkerEvent =
   | "error"
   | "pause";

-export class Worker extends EventEmitter {
+export class Worker extends SubscribingEventEmitter {
   constructor(options, jobs = {}) {
     super();

[glensc]

@evantahler let me know what you think of

• the naming of the class
• placement of the class in the directory

I will definitely need help on:

• typing (I used :any and ts-ignore for now)

[glensc]

and test cases to cover.

currently, I didn't test how this is passed to event handlers, can do this later if the approach gets a "go", not rejected.

[glensc]

@evantahler added more tests:

• class method
• static method of a class

can you help me with typescript types?

[glensc]

@evantahler added commit, as seems you don't understand how this all is glued together:

• 9078f94 Update worker to extend from SubscribingEventEmitter

[glensc]

@evantahler you never commented this PR but now just closed?

[evantahler]

I thought this was coupled to #335? If it's not, can you please provide an example to show how it would be used?

[glensc]

@glensc please read the pull request and comments and diff, it's all there including example and tests. and the usage you can read from #335 as well.