Sonobuoy CLI should allow specifying a directory of plugins

[johnSchnake]

Describe the solution you'd like
We currently allow sonobuoy run --plugin but if you have numerous plugins this can get tedious and is more frustrating to script around.

I'd like to be able to specify something like sonobuoy run --plugin-dir or --plugins which may take a directory.

I think this would just be a shortcut for adding all those plugins one-by-one and wouldn't require much effort.

Anything else you would like to add:
I don't think this has to be overthought or have complex interaction with the existing --plugin flag. I think it is reasonable to allow both to be set and they are all just additive (i.e. it would be valid to say --plugins lotsOfPluginsInThisDir --plugin alsoAddThisOne.yaml

I am hitting this because of the integration suite we are putting together and I am currently creating lots of yaml plugin definitions and want to run them all.

[zubron]

I have a slight preference here for --plugin-dir, just because of how similar --plugin and --plugins are.

[johnSchnake]

I don't think it would be hard to just change the flag handling to even let --plugin just handle files or directories. If it is a directory it will just load all the *.yaml files it finds in that directory.

It will prevent a new flag from being needed and just expands the use of that flag in a pretty resaonable way IMO.