"update" subcommand and tests in general

[ugandalf]

Resolves #70. It prints an error and then prints the general error help, but I feel like it should print the contents of cargo supply-chain help update instead (showing that it doesn't accept them), but that requires some restructuring of the code - possibly custom Error class altogether? I'd also like to add some tests, but not sure where to put them.

[Shnatsel]

Unit tests generally go in the same file. See:

• https://doc.rust-lang.org/rust-by-example/testing/unit_testing.html
• https://doc.rust-lang.org/book/ch11-03-test-organization.html

And yes, we'll probably need a custom Error struct for reporting errors. This was bound to happen eventually; the use of std::io::Error was really just a shortcut in many cases.

[Shnatsel]

I'm going to go ahead and merge this, because I am going to touch argument parsing and don't want to cause merge conflicts