Increase `STDERR` verbosity of CLI commands

[Ocramius]

While the commands are tested, we still cannot consider this "battle tested": we need to add more STDERR output to see what each command is doing, and when.

This is effectively just to aid downstream consumers in identifying any issues encountered during action execution.

[Ocramius]

Probably best to add a PSR-3 logger and pump stuff in there - many sub-services do a lot of stuff that we really want to know about (especially executed CLI commands and HTTP requests)

[weierophinney]

As discussed in Slack:

• PSR-3 logger as type injected in class constructors.
• Use Monolog with STDERR logger, configured in bin/console.php.

I'll get this setup initially, and wire the various keep-a-changelog functionality with it as a proof-of-concept, and then we can expand to other classes as needed.