Support --json for pnpm outdated

[esatterwhite]

npm supports json output for the `outdated command, pnpm should also support this kind of output

  "sinon": {
    "current": "8.0.0",
    "wanted": "9.0.2",
    "latest": "9.0.2",
    "location": "node_modules/sinon"
  }

pnpm version: 5.2.6

Code to reproduce the issue:

npm outdated --json

Expected behavior:

Actual behavior:

Error is thrown

 ERROR  Unknown option: 'json'

[kaiyoma]

This would be very useful for dependency upgrade automation.