Add: async creation of items

[andrewlinfoot]

This is a WIP to tackle issues: keystonejs/keystone#1705, keystonejs/keystone#1713, and keystonejs/keystone#1714. I'd love to get some feedback on it.

I just implemented async validation and left the .updateItem method alone. If this approach looks like the right direction, I'll make the updateItem methods async as well. From there, it should be easy to add the .processUpdateData api and handle file uploads.

A couple questions about the API in general:

1. Should the .validateInput api return more information than just false when there is an invalid input? I'm having trouble seeing how we would implement more granular error messages with the API as is.
2. Should the files be passed to the .validateInput method too or should file validation happen somewhere else?
3. Is it necessary to pass required to .validateInput(data, required, item, callback) when the field has the property .required and it can be accessed with this.required in the .validateInput method?

Overall, this new API seems way cleaner than the UpdateHandler approach.

[andrewlinfoot]

I submitted a different PR that made all of the updateItem methods async. That is merged in and good to go (keystonejs/keystone#2147).

I have some free time tonight and tomorrow and am planning on tackling the .processUpdateData API and polishing off the new update and create API endpoints. I would love to get some thoughts on this PR before I jump too deep into implementing the rest of the validation errors and adding support for file uploads. @JedWatson @morenoh149 @creynders @webteckie @ericelliott any thoughts?

[andrewlinfoot]

Just pushed some updates that implement a processFormInput method and change the validateInput method to call its callback with an error object instead of just true or false. This seems like a pretty solid API and should allow for file uploads, async data updating, async data validation and fine grained error messages defined by the FieldTypes themselves. Assuming I can get some consensus around this API, I'm going to start implementing processFormInput methods for some of the types that depend on file uploads. @JedWatson thoughts?

[andrewlinfoot]

Commit keystonejs/keystone@a73d131 adds a minimal version of processFormData for CloudinaryImage types

[andrewlinfoot]

Newest commits by Jed implement this API in a cleaner way. Closing the PR.