This prompts the user to enter a passphrase if the given PGP key is
encrypted.

Closes #1447

This removes the requirement that a fetch or install command must
explicitly state the version number to install. Instead, this goes to
the strategy used by OS package managers: Install the latest until told
to do otherwise.

Closes #1198

This adds the --verify and --keyring flags to:

helm fetch
helm inspect
helm install
helm upgrade

Each of these commands can now make cryptographic verification a
prerequisite for using a chart.

This adds support for OpenPGP signatures containing provenance data.
Such information can be used to verify the integrity of a Chart by
testing that its file hash, metadata, and images are correct.

This first PR does not contain all of the tooling necessary for
end-to-end chart integrity. It contains just the library.

See #983