Using a custom OAuth Client

[sboardwell]

(Note: Non-breaking issues are likely not to be prioritized. Please consider a PR in addition to your issue)

Question

With respect to #85, and in particular this comment would it make sense to make the clientID and secret configurable?

The idea would be to allow corporations to create their own trusted client and pass the clientId and clientSecret to the clasp command, either through parameters or the .clasprc.json file.

Would it be possible to get the steps necessary to create the client (the list of permissions)?

I would be happy to create a PR if you point me in the right direction.