Allow passing Ecto query and preloads as list_... function args

[vincentvanbush]

Closes #44.

In CRUD function generation, it will now allow calling generated list_... functions in the following ways:

• list_items() - lists all resources unfiltered
• list_items(query) - lists all resources filtered by the query
• list_items(query, [:category, :manufacturer]) - lists all resources preloaded with the associations
• list_items([:category, :manufacturer]) - lists all resources preloaded with the associations

Preloads can be given as an atom (:category) or a list of atoms ([:category, :manufacturer]) just as in Repo.preload/2.

The Ecto queryable argument must be either:

• the schema of the resource, or
• a query with from clause that targets the schema of the resource.

For example, list_items(Item) can be called just as well as list_items(from i in Item) - these are the most commonly occurring implementations of Ecto.Queryable that make most sense when used with a specific schema.