Add option to pass an existing executorService and or retrieve the last used

[icarocd]

The creation of the executorService, and its shutdown, requires an extra overhead which can be avoided while using the API in a program more than once.
Besides a collection to iterate in parallel, please add an option to receive an existing executorService, and not shut it down in this case.
PS: if it helps, I have this implemented already, although in this implementation I decided to split the classes from one file to one class per file.