Add --json support to 'heroku buildpacks'

[edmorley]

This project is for the Heroku CLI only and issues are reviewed as we are able. If you need more immediate assistence or help with anything not specific to the CLI itself, please use https://help.heroku.com.

Do you want to request a feature or report a bug?

Feature

What is the current behavior?

$ heroku buildpacks --json
 ›   Error: Unexpected argument: --json
 ›   See more help with --help

What is the expected behavior?

That the buildpacks command support outputting the buildpacks list in JSON form, since parsing the standard text output of heroku buildpacks is fragile, since one has to strip out the numbering, and there is no guarantee that the format won't change in the future:

$ heroku buildpacks
=== map-test-app Buildpack URLs
1. heroku-community/inline
2. heroku/ruby

...and it would be handy in certain use-cases to be able to just pipe the output of the CLI to jq without having to fall back to the platform API.