Add support for writing any reporter output to a file in addition to the
reporter on `stdout`. The `json` reporter is the primary use case.
Adds a `--file-reporter` CLI argument, and `file_reporters` yaml config
option.

- Add a `MultiplexingReporter` to allow wiring up multiple reporters to
  listen at once.
- Plumb through the configuration mapping a reporter to a file path.
- Instantiate each file reporter with a sink from the file instead of
  the normal `stdout`.