Make maximum number of items per page configurable

Currently the maximum number of items per page is hard coded in Krill, both in [KrillMeta](https://github.com/KorAP/Krill/blob/master/src/main/java/de/ids_mannheim/korap/KrillMeta.java#L24) and in [Result](https://github.com/KorAP/Krill/blob/master/src/main/java/de/ids_mannheim/korap/response/Result.java#L44). It should be modified by a configuration option.

(Suggested by @gremid)