Enable query diffing

[stubailo]

Working on implementing #42, looking for feedback on the design there! Not ready for feedback on the implementation though.

Todo

• Make query diffing actually output one query with all of the different nodes required
• Make query diffing use variables where the original query used variables
• Call the query diff function inside QueryManager when the appropriate option is passed
• Make sure to handle the case where the server doesn't implement the Relay ID spec

[stubailo]

Going to merge this and file some issues for the TODOs.

[stubailo]

Sigh, tired of waiting for AppVeyor.

[jbaxleyiii]

@stubailo yeah 👎 I've found appveyor to be really slow. I wonder if we can make that one CI not required? Or remove it?

[stubailo]

Ooh, maybe we can set it only for master. Let me check.

[stubailo]

OK it should only build master now. So we'll find out after the fact if we broke Windows, which is fine since it will probably be relatively rare.