"Chained" Task Type

[natsukagami]

This is to support the "Communication/Two-step" task type of CMS.

We would support a model like:

input -> ./code 0 -> ./code 1 -> output

In other words, a chained pipe of instances of the compiled binary.
This can simply be activated by uploading a file named .stages onto the list of problem files, with each line listing the argument passed to the binary.