Feature: implement login cache

[ewindisch]

The idea of 'iopipe login' is that it should take HTTP Authentication/Digest or perform OAuth authentication and cache the appropriate tokens within the .iopipe directory.

The Node SDK must be able to read this cache to perform authentications as well, but does not necessarily need to write it.

Finally, consideration should be given long-term to how authentication is dealt with for subscribed pipescripts, how we export, etc. It may be something that is not exported by default, but perhaps authentication tokens can be added to an export via a flag.