Feature Request: Diff to compare rollback

[codenio]

This provides the ability to visualise what a helm rollback could change.
Usage:

$ helm diff rollback -h
This command compares the laset manifests details of a named release 
with specific revision values to rollback.

It forecasts/visualizes changes, that a helm rollback could perform.

Usage:
  diff rollback [flags] [RELEASE] REVISION

[codenio]

This PR is now merged with #22 , closing this PR