Allow depth for at-slector

[kokorin]

We want to clone a subset of models form DEV schemas to developer's personal schemas.
For that we tried to use dbt clone -s '@state:modified' --state dev_artifacts. Unfortunately @state:modified selects all parents of children. So if we change a model from marts, almost every model is selected.

So I think it can be useful to allow specifying depth for at-operator like 1@state:modified.