pkgr plan as different aggregate output types

[dpastoor]

For example, would like to have

pkgr plan --output=json

{
  Repos: [
  ],
  packages: [
   {
     name: dplyr,
     version: "0.8.3",
     Repo: "MPN",
     Type: "Source"
   },
 {
     name: tidyr,
     version: "1.0.0",
     Repo: "MPN",
     Type: "Source"
   }
  ]
}

This would allow us to use plan as an "analytical" tool to make decisions without having to hunt through log files.

Granted, we can parse this out from the larger form logs by pushing out to the logfile with the individual json elements, but having a single parseable piece of json would be very nice.

What I don't know is if we can surface all this from a global object, or if some of those plan logs come from logic that is only available within those individual functions.