Enable specifying which credential set to use for a specific call

[kenperkins]

For example, I might have three different compute creds in my pkgcloud-cli.json file, but the helpers are hard coded to always use [0]:

https://github.com/pkgcloud/pkgcloud-cli/blob/master/helpers.js#L41-L49

We should make this accessible from the command line. Perhaps make them keys instead of arrays, and provide the optional keyname from the commandline (defaults to first perhaps)

This would allow you to have support for dozens of providers/regions.

[rosskukulinski]

great idea - I was talking with rdodev about that a few weeks ago. Its a breaking change, but I think the user base is small enough that we wouldn't piss too many people off.