Feature: allow editing the package.json in the `.tgz` package during `postpack`

[roydukkey]

Should be able to target the package.json location based on the process.env.npm_lifecycle_event == 'postpack', therefore not requiring any new APIs. However, research needs to be done in order to determine if changing the package.json in the tgz and during this npm_lifecycle_event break anything.

[roydukkey]

This is posing more of a challenge than I anticipated. It's been quite hard trying to figure out how to read and recreate the archive after cleaning. I've not even gotten to a point where I could test whether we could even intercept the npm pack build product in this way and insert clean-package's changes before publish.