provide utilities for custom checkers

[xiaowuc1]

Custom checkers are often error-prone because the writers are not sufficiently careful in handling malformed output.

This issue is a catch-all for utilities that can be provided to make custom checkers easier to write and result in fewer IEs.

[kiritofeng]

Idea: we create a Parser class that implements the reading functionality of dmoj.graders.interactive.Interactor and ship this class in dmoj.checkers.utils