Enable passing multiple input files to rdf to-jelly and treating them as separate frames

[adanilenka]

Currently, only one input file is allowed to rdf to-jelly.

For reasons, including proper frames division testing, it is required that there is a possibility to process multiple input files and assign each of them to a separate frame(or frames(?)).

Similar to the rdf transcode:
cat in_*.jelly | jelly-cli rdf transcode

It should also not contradict the --rows-per-frame option.