Change CLI config logic to support usage without environments

Scenario 1: Environment is defined via --env or NODE_ENV
* config is config[environment]

Scenario 2: Environment is undefined but config.development is an object
* config is config.development

Scenario 3: No environment and no config.development
* config is config

Also logs out the environment in use for scenarios 1 and 2
Add documentation for using CLI and knexfile without environments
Merge pull request #527 from tgriesser/cli-without-env

CLI support for knexfiles that do not provide environment keys