Read token credentials from a file

[ericyan]

We should provide a way to specify token credentials and other options using a file, like the OpenStack command-line clients.

[ericyan]

To clarify, we do have a config file, but that's primarily for CFSSL. We may reuse the file and add a token section to it.

If the PIN is not given, we may ask for it interactively.

[ericyan]

Alternatively, we can read from .env in the current working directory and fallback to ~/.config/lorica/loricarc.