Handle multiple -p port arguments

[gpittarelli]

Issue: Currently, if you run start-storybook and specify the port with -p multiple times, it errors out with Error: port to run Storybook is required! This is due to a quirk of how commander passes multiple args to the option formatter parameter, leading to parseInt getting called with a weird 2nd parameter and ultimately program.port being set to NaN. See, for example, tj/commander.js#523

This bug probably came up because the example on commander's README erroneously uses parseInt in this same way.

This is annoying when I have multiple checkouts of a project using storybook with the automatically added npm script: "storybook": "start-storybook -p 6006". If I want to run multiple storybook instances, I would expect to be able to do npm run storybook in one checkout, and then npm run storybook -- -p 6007 in the other to override the port number (It would expand to start-storybook -p 6006 -p 6007)

Most CLI programs interpret extra instances of an argument to override previous instances. For example: ls -l --block-size=1 --block-size=16 uses 16 as the block size.

What I did

Changed the commander option parsing function to pass only the first parameter to parseInt and call it with a specified radix.

How to test

Get this branch's start-storybook linked into some project with storybook setup, then start-storybook -p 6006 -p 6007 should start storybook running on port 6007.

[Hypnosphi]

I'd prefer to do something like what create-react-app does: https://github.com/facebookincubator/create-react-app/blob/master/packages/react-dev-utils/WebpackDevServerUtils.js#L380

Basically, they just prompt with suggestion for another port if the default one is unavailable

[Hypnosphi]

This particular PR should be merged anyway because passing parseInt as a function argument almost never is what you actually want to do.