text: Title: Change API spec file default name
  Body: Current the default name for the API spec is `api.yaml`. One suggestions is to change it to something more common in the industry like `Scanfile` (similar to [Fastfile](https://docs.fastlane.tools/advanced/Fastfile/#fastfile) or [Dockerfile](https://docs.docker.com/engine/reference/builder/))
  
  https://github.com/scanapi/scanapi/blob/master/scanapi/settings.py#L10
  
  Besides we also need to:
  - Update documentation at [scanapi/website](https://github.com/scanapi/website/tree/master/_docs_v1) 
  - Update examples at [scanapi/examples](https://github.com/scanapi/examples) 