More lenient parsing of rustc arguments

[Shnatsel]

Do not assume that if --crate-name is present, --out-dir must be present also. Instead allow them to be absent at the parsing stage, run the full detection logic for non-compilation commands, and only error out of the command looks like a compilation one.

Fixes #198

[benaryorg]

I can confirm that it manages to build the project I initially encountered this with (419 webserver-ish dependencies plus the crate itself) just fine on first try.
To be honest I have no idea how to test whether the resulting "auditable" part still works, however the built application works just fine.
If basic functionality of cargo-auditable works in tests though, chances are it works in the wild too, so just checking whether something compiles will hopefully suffice.

Regarding the "testing it on lots of real-world projects would be welcome" from the issue; I'm trying to check this with a CI pipeline, but I'm neither sure whether it'll work, nor how long it'll take so I'm probably of little help on that end.

[Shnatsel]

I've built 1650 randomly selected binary and cdylib crates from crates.io on this branch and that didn't turn up any issues. I'll do a more in-depth run overnight.