Support queries for preload

[liveforeverx]

For example: http://hexdocs.pm/ecto/0.15.0/ in Ecto.Repo.preload - preload supports quering.

We need to support query for preloading too.

Example in abstract form: get city load: weather, where weather.temp_lo > 30

Starts with:

1. propose API, that should be with some examples.

[0xAX]

@liveforeverx If we will use this variant:

get city load: weather, where weather.temp_lo > 30

How do we distinguish query for preload and main query? What do you think about:

get exd/city load:"weather => where:weather.temp_lo > 30, another_assoc" where:"name == \"Moscow\""

In this way we can parse command line option and its clear where preload query and where is general query. What do you think about this?