Add a value formatter prop.

[robinmacharg]

This change adds an optional formatter prop to the <InputSpinner/> component to allow users to better control the output. This can be useful in cases where direct numeric output is not suitable. Examples include e.g. converting time (seconds) to minutes and seconds, outputting non-decimal values (octal, hex etc), or cycling through enum values ('first', 'second', 'third'), all while maintaining the underlying numeric value.

Usage (Typescript):

<InputSpinner
    max={1000}
    min={0}
    step={5}
    value={seconds}
    formatter={(value: string) => {
      let totalSeconds = parseInt(value);
      let seconds = totalSeconds % 60;
      let minutes = Math.floor(totalSeconds / 60);
      return `${minutes}m ${seconds}s`;
    }}
/>