feat: add support for json flag

[mgi388]

Fixes #217.

[bnjbvr]

Thanks, looks good. One small comment and one question, and this should be good to go!

[bnjbvr]

Are these if !args.json required? The fact the tool is outputting with eprintln means this is going to stderr, and a user emitting JSON would likely be fine piping from stdout, so I think we could keep the stderr output as is. Thoughts?

[mgi388]

I went ahead and made this change (and have pushed the commits), however I think we want to go back to something like I had originally. Look at: https://doc.rust-lang.org/std/macro.eprintln.html

Use eprintln! only for error and progress messages. Use println! instead for the primary output of your program.

When not in --json mode, the primary output is what it used to print, and so that should use println!. So even

eprintln!("Analyzing dependencies of crates in this directory...");

Should change to use just println! according to that.

Now, if we do that, we cannot output both JSON and plain text, so I think I'll need to make a change to make it either or.

1. If --json flag is on, then it will only print JSON to stdout (using println!).
2. Else, then it will only print the plain text it previously printed to stdout (using println!).

I will wait to hear back before I make this change though.

[mgi388]

Note: Printing the version should go to stdout, so this change is wrong and will need to be reverted one way or another in this PR.