Take `impl Into<RepeatCount>` for easier usage (#78)

Change the signature of `Tween::with_repeat_count()` to take an `impl
Into<RepeatCount>` instead of a `RepeatCount` value, to make it easier
to configure the `Tween`.

Implement for `RepeatCount`:
- `From<u32>` for an actual count, yielding `RepeatCount::Finite(value)`
- `From<Duration>` for a duration "count", yielding
  `RepeatCount::For(value)`