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add support for running tests in parallel mode

> (this PR depends on most other PRs linked to #4198, so they should be merged first; documentation will be in another PR)

This PR adds support for running test files in parallel via `--parallel`.  For many cases, this should "just work."

When the `--parallel` flag is supplied, Mocha will swap out the default `Runner` (`lib/runner.js`) for `BufferedRunner` (`lib/buffered-runner.js`).

`BufferedRunner` _extends_ `Runner`.  `BufferedRunner#run()` is the main point of extension.  Instead of executing the tests in serial, it will create a pool of worker processes (not worker _threads_) based on the maximum job count (`--jobs`; defaults to `<number of CPU cores> - 1`).  Both `BufferedRunner` and the `worker` module consume the abstraction layer, [workerpool](https://npm.im/workerpool).

`BufferedRunner#run()` does _not_ load the test files, unlike `Runner#run()`.  Instead, it has a list of test files, and puts these into an async queue.  The `EVENT_RUN_BEGIN` event is then emitted.  As files enter the queue, `BufferedRunner#run()` tells `workerpool` to execute the `run()` function of the pool.  `workerpool` then launches as many worker processes are needed--up to the maximum--and executes the `run()` function with a single filepath and any options for a `Mocha` instance.

The first time `lib/worker.js` is invoked, it will "bootstrap" itself, by handling `--require`'d modules and validating the UI.  Note that _reporter validation_ does not occur.  Once bootstrapped, it instantiate `Mocha`, add the single file, swap any reporter out for the `Buffered` reporter (`lib/reporters/buffered.js`) then execute `Mocha#run()`, which invokes `Runner#run()`.

The `Buffered` reporter listens for events emitting from the `Runner` instance, like a reporter usually does.  But instead of outputting to the console, it buffers the events in a queue.  Once the file has completed running, the queue is drained: the events collecte... (continued)

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