Aynchronous request modification

[DarkDust]

This is a proposal to solve #37 by adding a closure that may be executed asynchronously to modify the request before it's actually sent.

[apollo-cla]

@DarkDust: 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/

[DarkDust]

have signed the Meteor Contributor Agreement.

[martijnwalraven]

Thanks for the PR! I'm a bit preoccupied with GraphQL Summit right now, so unfortunately I won't have time to give it the attention it deserves until next week.

My thought is that it would be great to see if we can make this more generic with a design similar to Apollo Link. The concept of links is pretty powerful, and I've seen some great uses of it at the conference today, so I would really like to open up that flexibility on iOS as well.

I would also want to involve @MrAlek, because I know he has some use cases of his own that he would need support for.

[designatednerd]

Hey @DarkDust - Thank you for doing this, and sorry for the long radio silence on our end!

Unfortunately since so much has changed since this PR was opened, I'm going to close this in favor of the work happening in #602 to implement this feature. Please feel free to chime in there if you have thoughts or questions, and open issues on anything you feel we may be leaving out.