Allow root_value to be passed along to GraphQL.execute

[aweiker]

During plug initialization, allows a function to be defined that will
take a conn as input and then return a data structure that can be
used as the root_value into the GraphQL.execute method.

[joshprice]

The callback needs to be in the same {module, function} format as the schema otherwise Phoenix code reloading messes with it.

[aweiker]

A note regarding this, currently the function is being pulled out and executed in the init function for grabbing the schema. For this function, I will need to make the evaluation and execution happen in call instead. Otherwise, we would never execute it per request or get the conn reference.

[joshprice]

Thanks for the PR! See comments, but otherwise looks great.

[aweiker]

I'm adding the comments and have most of them addressed. Do you prefer getting it rebased into a single commit, or do you want to see a new commit?

[joshprice]

Not too fussed about squashing commits, but this will need to be rebased on top of the new changes. Sorry I couldn't get this merged in before I refactored things. Ping me on Slack if you need a hand or have any questions.

The other thing that is missing is variables, which is pretty much the same as root_value so if you feel like adding that, cool, otherwise it's easy to add once this is merged.

[joshprice]

just match nil no guard needed

[joshprice]

specify arity 1 in is_function

[joshprice]

Awesome thanks again!