(don't merge) Add a version of decodeFile with support for a !include directive

**NOTE: This is not meant to be merged as is but rather to serve as a basis for a discussion.**

We need support for includes in YAML files.  Our use case is to have credentials and other config values in one YAML file during development, but include the credentials from elsewhere on production.

It turns out that this can be trivial implemented on top of `yaml` (this PR includes some proof-of-concept code).

@snoyberg Do you think something like this is useful as part of `yaml`?  If yes, I would prepare a proper PR (probably adding it as `decodeFileWithIncludes` to `Data.Yaml`).

BTW: For the used syntax, @soenkehahn did some research on what others use and suggested `!include <filename>` (see https://github.com/zalora/yaml/issues/1#issuecomment-61438368).
It *might* be worth considering using `Set` or `HashSet` instead of `elem` here for efficiency, but I highly doubt it's necessary. Somewhat similar comment could be made about `notElem` below.