Migrate home-grown output logging to standard Python logging

[TobiX]

This includes multiple steps:

• [ ] Figure out "correct" mapping from old "levels" to Python logging levels
• [ ] Replace all calls to out.(warn|debug|info|error) with standard Python logging calls
• [ ] Find a replacement for out.pager()
• [ ] Find a good replacement for tracking the current "context"
• [ ] Replace init code (cmd.py) with standard logging configuration
• [ ] Fine-tune output format & colors (maybe use rich?

Mapping from current "levels" (see https://github.com/webcomics/dosage/blob/master/dosagelib/output.py#L29) to Python standard levels:

out. method	level	Prefix	Color	Python level
error	0	ERROR:	RED	logging.ERROR
warn	0	WARN:	YELLOW	logging.WARNING
info	0	none	normal	logging.INFO
info²	1			(define extra level 15 maybe?)
debug	2	none	WHITE	logging.DEBUG
debug²	3			(define extra level 5 maybe?)

² Called with different level then default...

out.exception writes one line using error (Level 0) and the backtrace with Level 1. Since the default for dosage is 0, normal runs only get the error message, adding -v raises the level to 1, which allows you to see the backtrace.

Is this a mess? Yes, this is a mess.