Add support for cmd and args when creating a new service

[duglin]

Knative, via yaml, supports specifying the cmd and args in the podSpec - kn should support this too since there are times when people may need to specify which exe in the image to run or what args to pass to the exe. Sometimes the default values in the image are not sufficient.

[samodell]

@duglin Should this issue be in the client repo instead? Doesn't seem like a docs issue if the functionality doesn't exist in kn yet. I'm closing this out, but if that's not the case, feel free to reopen here.

[duglin]

hmm not sure why I opened it here - sorry :-)