Support further installation methods besides Helm Repositories

[dgloeckner]

According to the official docs, the following methods exist:

• A chart repository (as we've seen above)
• A local chart archive (helm install foo foo-0.1.1.tgz)
• An unpacked chart directory (helm install foo path/to/foo)
• A full URL (helm install foo https://example.com/charts/foo-1.2.3.tgz)

I'm specifically interested in the 2 "local" variants mentioned above.

Looking at the implementation of InstallOrUpgradeChart, install and update in client.go we could provide a variant which accepts a local chart directory.

I can try to make contributions if there's a chance that this gets merged ;)

[dgloeckner]

This is actually working as expected. See the docu update in PR #36