Add conditional verbosity helping debugging

Sometimes, often, npm shitstorm happens and deployment fails. That's the moment you want npm to be more verbose, helping you finding the culprit.

I made these little changes so that:
- the VERBOSE environment variable makes npm always verbose if set
- the BUILDPACK_RETRY_VERBOSE is more subtle: it run npm as usual, as long as npm is happy. If npm failed, it relaunch npm in verbose mode

Set environment variables with the usual `heroku config:set KEY=value` command.
These two flags can be cumulated with no worries because `npm --verbose --verbose`works fine.

Hope you'll like it.