[WIP] Supprt from:to:step in reader

When I wanted to try the `nth` parameter I was very surprised about what it was actually doing. It did the same as the `offset` (I thought it was every `nth`-file)! I didn't notice that before because it sounded pretty straightforward... Sorry for that. Anyway, this is a proposal which breaks the API but would finally support the whole deal - from:to:step. There are questions which I will ask inline. Hopefully, this will fix it once and for all.