Update rubocop.yml configuration
Add Config Spec
Add Config module for setting api_key and options
Add ::Config to Filepreviews module
Add spec to configure the api_key header in the http module
Add configure_api_auth_header method to HTTP module
  Refactor :default_connection (header conf block)
  Add :configure_logger to clean-up :default_connection
Add functional specs to test more cases when using API key
  Refactor previous specs to clean-up
Add FILEPREVIEWS_API_KEY Env key for Travis
Add api_key argument and description to CLI
Update README with new API Key and Configuration examples
  Add url to Filepreviews.io for registering an App
  Add command-line usage examples
Merge pull request #3 from jonahoffline/add_api_auth

Add api authentication