Add `StdInOpener` as flux modul `open-stdin` based on org/metafacture/io/StdInOpener.java

[TobiasNx]

While writing to stdout is possible with print, opening/reading stdin is not possible with flux.
Therefore I suggest (inspired by a chat discussion with @blackwinter ) to introduce open-stdin as Flux module.

There already exists a java class for this, which cannot be used in a Flux yet:
https://github.com/metafacture/metafacture-core/blob/master/metafacture-io/src/main/java/org/metafacture/io/StdInOpener.java