[Draft] Use builder pattern and enable stream selection

[EvysGarden]

I've added a builder and removed all decoder::new... functions. The builder has a function DocoderBuilder::reader_stream_index(selector: SelectorFn) which can be used to select a stream as requested in #31 .

Example

let mut decoder = DecoderBuilder::new(&source, None)?
    .resize(Resize::Fit(400, 400))
    .reader_stream_index(|_| 6)
    .build?;

What needs to be done

• Add documentation

[gerwin3]

closing since this will be part of #37