Feature/update priority

[RaishavHanspal]

#208 - add a uid to track the promise functions and update the priority on any promise.
also add a new event started to notify when a promise function is executed.

[RaishavHanspal]

requested changes have been pushed! @Richienb

[Richienb]

I think we should allow searching by the object returned by enqueue as well. What do you think? Does there remain a use case where a string id remains more useful?

[RaishavHanspal]

@Richienb that would be a very nice idea, and will not require id property. The problem that I faced was enqueue accepts RunFunction - which in my understanding is a callback not accessible to the end-user.
The Idea of priority update - is for the user to be able to update the priority on the fly, currently this is achieved by a unique id assigned by user, or this can be passed using the invoke event as added in #210 incase assigned by #idAssigner.

[RaishavHanspal]

@Richienb, @sindresorhus , Please suggest further.