Add configuration

[mrcnski]

Entering the same options e.g. --api-key every time can be tedious. There should be configuration options such as a dot-file and/or environment variables. Some initial thoughts:

1. As part of Smart portal selection skynet-docs#21 we should support system-wide configuration files with not just default portal selection but also connection options like --api-key etc.
2. We should also support environment variables as they are standard fare for cli applications. They should be namespaced with SKYNET_ e.g. SKYNET_API_KEY.
3. The priority of configuration should be as given here, that is, check first for cli options, then env vars, then config options.

Note: it is actually preferable to outsource these configuration responsibilities to the Go SDK itself. Every call from the CLI to the Go SDK will initiate a new client which should initialize the appropriate values based on the configs.