Auto outside pkg

[eteq]

At the request of @ChrisBeaumont, I've re-written the part of #1801 that allows using automodapi to document things that are not actually in the package you're documenting.

This is useful for "namespace modules", e.g., where you have something like:

implementation1.py
implementation2.py
namespace.py

where namespace.py is something like:

from implementation1 import *
from implementation2 import *

If you just do .. automodapi:: namespace in master, nothing will appear, because implementation1 and implementation2 members aren't included. But after this, if you do:

.. automodapi:: namespace
    :allowed-package-names: implementation1, implementation2

Anything that's acutally in implementation1 will show up on the module summary.

[ChrisBeaumont]

I finally had a chance to test this out, and it's great! Thanks for putting this together.

[eteq]

Note that the sphinx build failed, but because of an unrelated problem in master (fixed in 8d45943)

[embray]

Sounds good to me. Just as a reminder to myself, this will have to be pulled into astropy-helpers as well. But it should also be here for backward-compatibility.

[eteq]

I'm going to rebase this against master just to make sure the sphinx build passes against the latest master, and if so I'll merge this, then.

[eteq]

Hmm, an interesting quirk of Travis that I had not noticed before: If you use [ci skip], apparently it will skip even if you later rebase. So I'll just put in a quick no-op commit to get travis to run.

[eteq]

Alright, it's now actually passing, so merging!