Clarify the CLI flags

[Nadrieril]

We've accumulated some cruft I'd like to clear up.

• the various --print flags could all be a single --pretty-print;
• --ullbc could be --ast=ullbc or --output=ullbc (see also Feature request: translate to MiniRust #1251 (comment));
• --extract-opaque-bodies is weirdly not exactly --include=*;
• --translate_all_methods should be --include='trait **' or something;
• --dest/--dest-file is an unconventional name;
• --abort-on-errors/--error-on-warnings always confuse me; maybe --error-level=warn/err/panic.