Added support for 'helm diff upgrade --install'

[MFAshby]

This behaves exactly the same as 'helm diff upgrade --allow-unreleased',
but it matches the same arguments as helm itself, so that check scripts
and deployment scripts can be basically the same, e.g.
check:
helm diff upgrade --install myapp ./myapp
install:
helm upgrade --install myapp ./myapp

and this'll work regardless of whether the release exists or not.
Ref #108

[mumoshu]

@MFAshby Thanks! LGTM.

Note that this doesn't resolve the original issue, so I'll reopen the issue once this gets merged.
But this feature itself should be great to have! Thanks again for your contribution.