Adds JSON output to `outdated` command

After some musing on brunophilipe/Cakebrew#71, I thought it would be useful to let Cakebrew and other tools grab the outdated formulae version information using a method more elegant than regex.

I poked around quickly for a pattern for how other commands are tested, but couldn't find it in the time I have available. I'll happily add a test if you can point out where!

I'm not entirely set on the keys in the JSON output. I went with the internal terminology, but think that `name`, `installed`, and `available` would be more expressive.

I'm pretty sure I got all the other things, too, like completions and man page.