Add support for formatted query output.

[DaveRMaltby]

For a project that I'm working on, at runtime, I'm generating GraphQL queries that are displayed to the user. Therefore, I wanted the output of your library to contain appropriate line breaks and indentations. I've accomplished this on my own fork, but wondered if there is any interest in me providing a PR here to add this optional formatting to the output. Thanks.

[charlesdevandiere]

Thank you @DaveRMaltby, it's a good idea!