logging fixes/documentation

[jdx]

We need some things with logging:

• [ ] fix the --log-level cli flag (it doesn't work at all right now). This is actually a really hard change but must be done. I think as a further improvement we can use this: https://crates.io/crates/clap-verbosity-flag. That's a lot friendlier IMO.
• [ ] document environment variables
• [ ] switch to paris: https://github.com/0x20F/paris (it looks like simplelog doesn't allow customizing the output very well, and this has some great functionality for spinners and colors that I think would clean a lot of things up). It may be best to use both actually, simplelog can use paris internally and I think simplelog has features we want (file logging and source line no) that I don't think paris can do.

The second is something easy for somebody else to help out with, we have RTX_DEBUG=1, RTX_QUIET=1, RTX_LOG_LEVEL=trace|debug|info|warn|error. That just needs to be added to src/render_help.rs so that it goes into the readme.