There are some cases where the existing module has difficulty
determining if an upgrade would result in changes. This can particularly
be a problem when changes are made to a local chart.
This adds optional support for helm diff. If the plugin is present it
will be used. Otherwise, the default implementation will be used and a
warning will be issued. One caveat: helm diff does not currently support
using a repo url, so the default implementation will be used in this
case, as well.
Closes: #248
