Add a --confirm-prompt option to support prompting if --confirm missing

[gpoole]

Thanks for the handy plugin.

I usually put my deploy command in my package.json's scripts section, e.g.:

"deploy": "serverless deploy" 

Doing it this way it's a little bit more awkward to pass the --confirm command (npm run deploy -- --confirm) so it'd be nice if I was able to set it up to prompt for a confirmation if a flag isn't provided, e.g. something like:

$ npm run deploy
Are you sure you want to run 'deploy'? y/N

I think the prompt would be off by default and enabled with a flag e.g.:

serverless deploy --confirm-prompt

Would you be interested in a PR for that feature?