#61 Add custom CopyBookResolvers

[FrankTheTank]

It would be nice if a user could dependency-inject a custom CopyBookResolver, that takes a Copybook-Name as an argument and returns the content of that particular copybook for further processing.

We would gain much flexibility therewith, because we could model our specific lookup logic.

[KrisDS]

First attempt in [r340].

Please try it out and let me know if anything needs changing.