Implement a plugin audit command about krew

[lbb]

/fixes https://github.com/GoogleContainerTools/krew-index/issues/15

[ahmetb]

I like the idea. I think it should not be a user-visible command that pollutes "kubectl plugin --help" surface. For example, brew audit is hidden by default. But I think we can't do that just yet.

How about making this a separate plugin that only plugin devs need to install? This way the audit can evolve/live independently.

On the CI side, I think we need a command that can download and install the plugin for ALL os/arch combinations. So something in the lines of:

<cmd> audit --source=<plugin>.yaml [--os=] [--arch=] [--target-dir=$KREW_INSTALL_PATH?]

[ahmetb]

So we solved the --os/--arch customization with KREW_OS and KREW_ARCH in #70. Similarly, --target-dir= was solved in #65.

The primary challenge I'm facing right now is figuring out all the supported os/arch matrices from platform.selector to write a cmd that executes

KREW_OS=... KREW_ARCH=... KREW_ROOT=$(mktemp -d) krew install --source=<plugin.yaml>

to verify all platform urls/fileOps are working.

[ahmetb]

Postponing to after v0.2.

[ahmetb]

Fixed with #151 by adding a manifest validator tool.