This change adds a new configuration option to number formatters to have
spaces (or other strings) separating the formatted number from units

The new option is called `unitSeparator`, and it can be passed to
formatters that use units, for example:

```
// default behavior
expect(kilobytes(1, 0)).toBe('1.0kB');

// new behavior
expect(kilobytes(1, 0, { unitSeparator: ' ' })).toBe('1.0 kB');
```