Add a way to load collections in another bucket

If I have the following yaml file:

```
chefclub:
collections:
categories:
...
```

I'd like to be able to use `kinto-wizard` to load this file in `chefclub-preview` rather than `chefclub`

kinto-wizard load recipes.yaml --bucket "chefclub:chefclub-preview"

or

kinto-wizard load recipes.yaml --bucket "chefclub" --dest-bucket "chefclub-preview"

What do you think?

Right now I just do:

sed -i "s/^chefclub:$/chefclub-preview:/g" recipes.yaml

Start at the kinto-wizard load command and trace how --bucket selects the destination from recipes.yaml. Compare the two proposed command-line forms and confirm how collection paths are resolved; done means loading the file into chefclub-preview without editing the YAML while preserving existing behavior.

Written by the indexing model from the issue text.