Fix hanging of the interactive sample

[rexim]

Split from #281 (comment)

Steps to reproduce

$ sbt "run docs/samples/05-interaction.mgn"

Observed

Sample hangs indefinetly

Expected

Sample behaves according to the original sample description by @ForNeVeR:

Add a complex interactive example. Read two numbers from stdin and print their sum. Read a list of numbers from stdin (is it possible? Could Morganey program detect EOF? If no, then the length of the list should be the first parameter read), print their product.

#281 (comment)