Add concurrency option on forEach and map

[BenjD90]

Add possibility to limit number of concurrency requests on foreach and map

[toniov]

Thanks for the PR! This is a good feature I think, but I have some concerns about the implementation:

• You are adding the promise-pool-executor module, but I'd like to keep this library as simple as possible and dependence free at the moment. Also I took a look to the repo, and that module doesn't have been tested enough by the community maybe.
• Instead of adding a concurrency parameter, I'd prefer to add a general options object parameter, so in the future more options can be added.
• I don't think it's working properly. When you call pool.addEachTask all the promises are already being executed so there's not real concurrency control. Also the tests don't reflect if concurrency is working properly (I think that testing this could be kind of difficult).

[BenjD90]

• It's difficult to manage the promise pool I think, that's why I used another library to do that.
• Ok for the options, but it should merge with the thisArg parameter too, no ?
• You're completely right for the last point, I'll have to change that.

I don't have a lot of time right now to fix this 😞 I'll check that later.

[toniov]

I think that maybe adding this option without changing too much code is going to be difficult.

I'm planning to release a version 2, so I think introducing concurrency at that point would be better.

[toniov]

Closing this, thanks anyway :)