Hide unstable options from --help without -Z unstable-options

[joshtriplett]

To simplify --help, we should hide unstable options unless you pass -Z unstable-options --help.

[epage]

At this time, clap does not support dynamically generating help based on other flags being present. This would be made even more complicated by the fact that -Z is also a global flag.

As an alternative, we could check the channel and hide all unstable flags based on that. This would make it so they only show up when using a nightly build.