Addon-knobs: Allow text and number to take undefined values

[davidcsally]

This allows developers to pass optional props to text and number knobs
in storybook. A common pattern for this is when a React component has
an optional string or number prop.

Issue:

text() and number() knobs do not allow undefined values, which become many optional props when writing React components with typescript.

What I did

Update the types for each of these props to allow the value to be optional. I first tried changing value to optional inside KnobControlConfig (src/components/types/types.ts), but this threw errors in the Date knob, and I didn't want to dive into that.

How to test

This already works in knobs today when using typescript - if you pass undefined to a text() or number() knob, you will see a typescript warning, however, if you add a //@ts-ignore, you will notice that the knobs functions as expected with no value provided.

Here is a very simple code example:

This component (SimpleText) takes an optional value prop, which can be a string or a number. To represent this in knobs, you can pass undefined as the value (or you may omit it as I have done in the number example). However, value is a required property for both the number and text functions, which will generally throw typescript errors during compilation.

import React from 'react'
import { text, number } from '@storybook/addon-knobs'

interface Value {
  value?: string | number;
}

const SimpleText: React.FC<Value> = ({ value }) => <div>{value}</div>

export const emptyDiv = () => (
  <SimpleText />
)

export const textDiv = () => (
  <SimpleText value={text('Text for div: ', undefined)} />
)

export const numberDiv = () => (
  <SimpleText value={number('Number for div: ')} />
)

export default {
  component: SimpleText,
  title: 'Example|TextDiv'
}

[shilman]

LGTM!

[davidcsally]

Hey @shilman! Just wondering if I should rebase this so it is current with master.