Add a check command line param for CI

[dessalines]

This would be nice to have, then we could have it fail on our CI if someone added a dependency that is actually unused.

[bnjbvr]

Hi! cargo machete will return a status code if any unused dependencies have been found:

• 1 if there was at least one unused dependency
• 2 if there was an error during processing

Is this something you could use?

I think we could document this a bit better, and also just set up our github action so people can use this more easily.

[dessalines]

Ah okay, I didn't realize it returned proper status codes, that should work fine already then. Thanks!

[bnjbvr]

I've added #72 which tweaks the README to let users of machete know about the return code. I'll investigate the Github action later, would be nice to have too :)