Support custom schedulers

[leebyron]

Originally discussed as early as issue #1! DataLoader's primary value is providing performant coalescing batching. It does this by exploiting knowledge of how Node's job queue runs (and just using setImmediate in the browser). However many ports of this library have not found equivalent mechanisms in their host runtime and have instead opted for either a timer-based solution, manual dispatching, or tight integration into a custom GraphQL executor. All of these approaches are perfectly valid and it's time that the original implementation provide the capability to explore these mechanisms as well.

This PR allows overriding the built-in default enqueuePostPromiseJob scheduler with any arbitrary function. The Readme documentation has been amended to show examples of time-window collection (via setTimeout) and manual dispatch.

Closes #225