Make Dataloader optional

[RichAyotte]

Because of my ACL needs, I've had to implement a custom Dataloader.

Would you accept a PR to make the currently included Dataloader an optional feature?

[mickhansen]

I'd accept a PR to allow disabling it definitely.
Although i think it's a lot simpler to think about ACL post fetch most of the time.

[idris]

@mickhansen had an interesting idea here: #535 (comment)

But going forward, when dataloader implements per-request batching i'd love to just remove dataloader from here and just direct people to it in the readme.

Also related: mickhansen/dataloader-sequelize#39 (comment)

The library [dataloader-sequelize] really needs to be refactored so that it works off request caching rather than global caching.

I think that should be the direction here:

1. Fix dataloader-sequelize so that it is more manual and works off of request caching. It will require a dataloaderSequelize object to be passed in with every call, just like transactions.
2. Add an option (or options) in graphql-sequelize to create a dataloaderSequelize context and attach it to the graphql Context.

This way, if you don't want to use dataloader-sequelize, all you have to do is not add it in your graphql context.

One drawback: You will not be able to enable dataloader-sequelize for certain associations but not for others. This would be an easy fix, though.