Add json output to list command

[bartmeuris]

Addressing #141 - this adds JSON output to the list command when adding the --json flag.

I tried my best to keep the normal list output exactly the same as the last version available on homebrew:

$ displayplacer --version
displayplacer v1.4.0

Developer: Jake Hilborn
GitHub: https://github.com/jakehilborn/displayplacer
LinkedIn: https://www.linkedin.com/in/jakehilborn
Email: jakehilborn@gmail
$ displayplacer list > list_old
$ make && ./displayplacer list > list_new
$ diff list_new list_old
$

No attempt has been made to format the JSON output, I leave that to external tools like jq.

[milichev]

@jakehilborn, I'd be excited to hear about any progress on this feature