Make formatters configurable

[sanmai-NL]

Only use subprocesses for formatting, and allow configuration of the formatter's command line for each supported data format.

This allows for better integration in existing pipelines with standard and I dare say, more common/popular formatters.

[hukkin]

Only use subprocesses for formatting

I'd like to always avoid subprocessing if possible. It's slow, and requires dependencies that often can't be expressed in package metadata.

I'm not sure I want to add configurability to code formatter plugins I maintain. Should be fairly simple to create a fork if it's something you need.