How to specify the compression level?

I don't see any way of specifying the compression level in the encoder?

Another rust library allows specifying this as [the level config](https://docs.rs/lz4/latest/lz4/struct.EncoderBuilder.html#method.level)

```rust
    // 0 == default (fast mode); values above 16 count as 16; values below 0 count as 0
    level: u32,
```

How can I specify this in `lz4_flex`?