Logging levels to different outputs

I think it'd be a useful addition to allow logging different logging levels to different outputs. If this is already possible, please excuse my inability to search issues or read documentation or read code. Especially the last.

Self deprication aside, it's currently hardcoded what targets can be used with env_logger. The current issue I'm facing is the following:

• I log info logs
• I log error logs
• Both go to stderr, stdout or a pipe I guess

I don't like that, I'd prefer logging info to stdout and error to stderr. That should be relatively simple with a Mixed target or smtg, and implementing the different targets seperately instead of matching an enum. That'd even open up the possibility for custom targets!

Do I dream too big? Yes. But!

Okay, yes.

Thanks, hope this sparks some ideas, I'm sorry for being so casual k thx bye <3

(also, letting me implement it is a bad idea I cannot do the rust well)

Start by reading env_logger's current hardcoded target handling and the discussion around issue #364. Determine how a Mixed target, per-level routing, and custom targets would fit the existing target and enum design. Done means the behavior and API are agreed on and routing such as info to stdout and error to stderr is implemented and tested.