Add ability to load includes in yaml config

From commit message:

```
We often need to reuse configuration between apps, extracting those
reusable configs to a separate files will simplify making changes, such
as changing credentials.

This commit adds ability to include files into configs. Includes can be
both global and local for given env:

    includes: # will be included in all envs
      - pusher

    production:
      includes: # will be included for production only
        - encryption

Included files can contain configuration for each env separately, but
also specify config for all environments. For example sponsors.yml could
look like this:

    all: # will be included regardless of env
      sponsors:
        - sponsor 1
        - sponsor 2

And database.yml could look like this:

    production:
      host: some-host

    staging:
      host: other-host
```