Add formatter prop for InputNumber

[afc163]

react-component/input-number#47

Thx for @Jiramew

[mention-bot]

@afc163, thanks for your PR! By analyzing the history of the files in this pull request, we identified @benjycui, @yesmeck and @yiminghe to be potential reviewers.

[coveralls]

Coverage remained the same at 64.358% when pulling 08c5fe2 on improve-input-number into d09d667 on master.

[acomito]

This is a great update. I just grabbed 2.8 to use this in my project, it's easy enough to add a $ to the front, but I'm trying to do comma separators. This is my setup and the resulting behavior:

http://g.recordit.co/cfwQuZYcpY.gif

It seems to work when numbers are added from the front, but if you type number into the input with cursor at the back of the number, you get some issues. Does this issue seem to be with the formatter callback or with my comma function?

const numberWithCommas = (x) => {
    return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
}

<FormItem >
                  {getFieldDecorator('value', {
                    rules: [{ required: true, message: 'Please input your Value!' }],
                  })(
                    <InputNumber 
                      formatter={value => `$ ${numberWithCommas(value)}`}  
                      min={10000} 
                      max={10000000000} 
                    />
                  )}
                </FormItem>