Open up customization of resource requests

[mulhod]

Currently, we can only customize resource requests in a few different ways, such as specifying the number of GPUs, the host, etc. But this is only the tip of the full space of resources that could be made available for specification. In some cases, there are even user-/grid-specific resources, so making a new resource keyword (like we did for GPUs) is not the best approach. We should think about refactoring this so allow the user to customize the resource requests to be exactly what they need.

See #88