allow stdin/stdout markers (ex: "-") in place of files

[aryarm]

to allow for piping between commands
It would be cool if we could do this, for example:

stepik step https://stepik.org/lesson/9172/step/2
stepik dataset | tr ' ' '+' | bc | stepik submit

[aryarm]

In the meantime, using /dev/stdout and /dev/stdin seems to work

stepik step https://stepik.org/lesson/9172/step/2
stepik dataset /dev/stdout | tr ' ' '+' | bc | stepik submit /dev/stdin