Change API to take options object as a second parameter

### Why?
https://github.com/pwlmaciejewski/imghash/pull/78 changed the API slightly: it will always return a promise where previously it would throw early.

It's a breaking change so it's also a good time to update the API to be easier to work with.

### What?
Change `hash` signature to be:

```ts
hash(filepath: string, {
  format: string,
  bits: number
})
```
