Improve logging configuration

[hoelzro]

Instead of just providing a list of logger configurations, we should be able to provide shortcuts to common tasks. Like this:

log: '/tmp/filename.log'
log: '@screen'
log: 'mailto:me@example.com'

Also, we should allow more power for users that want it. I can see expanding the options like so:

---
log:
  timestamp_format: '%F %T'
  min_level: 'info'
  max_level: 'fatal'
  loggers: '@screen'

logger could also be an alias for loggers.