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

[blackwinter]

This is already possible in Flux with the > instruction (via StdInOpener).

>
| decode-xml
| handle-generic-xml
| encode-json
| print
;

$ ./flux.sh test.flux <test.xml

[TobiasNx]

I have to questions how would we pipe something into the flux:

1. How do you pipe something in echo '{ "test" : "case"}' | /flux.sh test.flux <????"
2. Do we intend to introduce the open-stdin command or is the >-instruction sufficient? I find it confusing with regard to the flux.

[blackwinter]

1. This is just regular shell syntax: echo '{ "test" : "case" }' | ./flux.sh test.flux (no need for < if you don't want to read from a file/process)
2. If you want to introduce open-stdin as a Flux command (essentially as an alias for >), you have to change the grammar accordingly. Strictly speaking, this would not be a command but a syntax element. (Actually allowing commands as root elements in the grammar is a whole different matter.)

[TobiasNx]

Okay thanks! Okay, I just wondering how we should document this so that this is not overlooked.