Add configuration

[mrcnski]

There should be configuration options such as a dot-file and/or environment variables. Some initial thoughts:

1. As part of Smart portal selection #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.

This applies to all SDKs except the CLI, which should outsource the work of determining configuration to the Go SDK. See NebulousLabs/skynet-cli#7.