Add Standard for code linting and formatting

[brettchalupa]

Tasks:

• add standard
• add minitest ext if needed
• address autocorrectable offenses
• address other manual offenses
• make default rake task run RuboCop for easy running w/ tests
• document its usage in CONTRIBUTING
• get it working on CI

[brettchalupa]

Having a heck of a time getting this working on CI due to unrelated require attempts within the RuboCop Rake task, see this branch: https://github.com/brettchalupa/graphql-docs/tree/rubocop

Putting this on hold as it's not a priority and is just nice to have for now