Proposal: Common Functions for output handling \ Image name parsing etc.

[simongdavies]

cnab-go should implement common functions that drivers can use to save each driver having to reimplement mostly the same functionality: Two specific examples:

1. Image Reference Parsing
2. Output validation and default value processing

[glyn]

Please note an existing dependency of cnab-go, github.com/pivotal/image-relocation, provides an image reference parsing package.