Added support for `--mode json-file`, removed all unwraps, added better error handling

[joshterrill]

Hello!

I'm using this binary in a project and saw that the docs say json was supported, but the code didn't actually implement it. So I've taken the liberty of doing so, as well as removing all of the .unwrap()'s where the program was often panicking due to errors, so there should be much better error handling now, graceful exits, and better messaging.

[joshterrill]

I also saw that there weren't any tests in the project, I was going to take some of the examples and turn them into rust tests so that it can easily be run through cargo test. I just didn't quite get to it last night.