Add description field to mutationWithClientMutationId

[christensenemc]

Adds an optional description field to the mutationWithClientMutationId function.

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[christensenemc]

resolves #57

[wincent]

Thanks for this, @christensenemc, and sorry about being slow to look at it.

I think this looks basically good, but I'll add a couple of comments inline in your commit after this.

[wincent]

This field is optional in GraphQLFieldConfig, so we should probably omit it rather than return an empty string.

[christensenemc]

Looks like travis is failing on this babel-eslint bug

[wincent]

Thanks for chasing down that babel-eslint issue! Good detective work.

I left one more comment inline, but this looking great.

[christensenemc]

Third time is the charm! It returns undefined on for field.description now if not in config. Better fits with the flow type, though personally I'm a little squeamish throwing around undefined like that.

[brad-decker]

Would love to have the description field for graphiql documentation and introspection.

[wincent]

Can we get a final rebase on this so that we can merge it without conflicts?