Use exit code to signal if there are unused dependencies

[Bromeon]

Addresses part of #13.

The application now returns 0 if no unused dependencies are found, 1 if unused dependencies are found, and 2 for any other errors (propagated via Result).

What could still be improved in a separate PR:

• Error handling (it looks like syntax errors in some Cargo.toml files are skipped instead of causing an error; not sure if intended)
• Machine-readable, stable output

[bnjbvr]

Nice nice, thanks!

[Bromeon]

Somewhat related, do you plan on releasing a new cargo-machete binary soon, which includes those semantics?
I plan on using it in CI, and having the exit code would help greatly.

If not, I'll go with cargo install --git for now; this might not work well with caching artifacts though.

[bnjbvr]

Yep that's a good idea, will do that in the upcoming days!

[bnjbvr]

And now done, published as 0.3.0!