Add OperationsMustHaveNames lint rule

[gauravmk]

Adds a lint rule that enforces all operations are named. See #42 and https://apollographql.slack.com/archives/eslint-plugin-graphql/p1488999103000014.

This adds it as a separate rule: graphql/named-operations so that it's easier for folks to configure it on or off / warn vs error. Did a little restructuring to be more DRY

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests pass
• Update CHANGELOG.md with your change
• If this was a change that affects the external API, update the README

[apollo-cla]

@gauravmk: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[jnwng]

this is a great contribution @gauravmk, thank you so much — i really appreciate that you took the time to make it extensible for additional validators in the future.

[jnwng]

@gauravmk do you mind amending the README to capture how to invoke this rule (and perhaps an example of what it lints against?)

if you're unable to do this, i can take a quick stab at this tomorrow.