Allow common cross-environment configuration

[janstenpickle]

Currently shanty only supports config per environment, but some plugins will require global configuration which exists across all environments. Propose something like this:

---
- all:
  - docker:
    - nic: cage
- local
  - docker
    - nic: coppola

[nathankleyn]

:+1:

Perhaps an all key rather than common? I hate the word common, it's overused. :-)

[janstenpickle]

:+1: Good plan

[nathankleyn]

Fixed by #36.