Graphql-compiler

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- Add GraphQL pretty printer: :code:`python -m graphql_compiler.tool`
`23 <https://github.com/kensho-technologies/graphql-compiler/pull/23>`__
- Raise errors if there are no :code:`output` directives within a
:code:`fold` scope
`18 <https://github.com/kensho-technologies/graphql-compiler/pull/18>`__

Thanks to :code:`benlongo`, :code:`ColCarroll`, and :code:`cw6515` for their
contributions.

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Initial release.

Thanks to :code:`MichaelaShtilmanMinkin` for the help in putting the
documentation together.