Feature request: Support for dimensions

Thank you for a great library!

It would be useful if the library supported higher dimensions for cases such as "m²" or "m³". An example would be:

```
humanFormat(1_000_000, { unit: "m²" });
//=> 1 km²
```

Or alternatively:

```
humanFormat(1_000_000_000, { unit: "m³" });
//=> 1 km³
```