[FEATURE] `bundle_report outdated` outputs in JSON format when passed optional argument

[boscomonkey]

Optionally specifying JSON as an output format will allow users to easily run bundle_report as a scheduled job (or as part of CI/CD) and alert on required upgrades.

For example: bundle_report outdated --json outputs

[
  {
    "gem": "rspec-support",
    "installed_version": "3.10.2",
    "installed_released_at": "2021-01-28",
    "latest_version": "3.11.0",
    "latest_released_at": "2022-02-09"
  },
  {
    "gem": "rspec-mocks",
    "installed_version": "3.10.2",
    "installed_released_at": "2021-01-28",
    "latest_version": "3.11.0",
    "latest_released_at": "2022-02-09"
  },
  {
    "gem": "msgpack",
    "installed_version": "1.4.2",
    "installed_released_at": "2021-02-01",
    "latest_version": "1.4.5",
    "latest_released_at": "2022-02-15"
  }
]