Allow additional metada fields when describing schema

[sgrove]

It'd be quite useful for us and some of the tooling we're building to be able to attach metadata to a field at definition time. This might be things like some arbitrary cost value, so that the tooling can sum up the cost of an entire query and alert the user in graphiql if it's exceeded their per-query quota (similar to the system described by @tgvashworth in https://youtu.be/Baw05hrOUNM?t=8m29s). Their might be lots of things we want to stuff in there to enable various kinds of tooling.