This adds support for installing CRDs well before any other resource
kinds are installed.

This PR introduces a new hook, `crd-install`, that fires before
manifests are even validated. It is used to install a CRD before any
other part of a chart is installed.

Currently, this hook is _only implemented for install_. That means we
currently cannot add new CRDs during `helm upgrade`, nor can they
be rolled back. This is the safest configuration, as the update/rollback
cycle gets very challenging when CRDs are added and removed.