Allow for json output when running cargo install-update -al

[mainrs]

Makes it easier to work with when calling from scripts.
If you're OK with it I can open a PR, doesn't seem to be that difficult to implement.

[nabijaczleweli]

Sure, open a PR, though I'm not sure what you'll gain from JSON specifically that you can't get out of the current output at present

[mainrs]

It's just easier to parse JSON in other languages than it is to parse some arbitrary text columns. awk is fine for what but if I want to use Python to parse the output JSON is more reliable :)