Support interactive checkers

[MartinXPN]

Sometimes the checkers interact with the program through stdin/stdout.
This might be a good feature for competitive programming style problems.

Requested by Armen Andreasyan.

[MartinXPN]

We should probably have 2 threads to read from the subprocesses. And there are 3 processes:

1. The main process (Coderunner) which is responsible for the communication between the other two
2. The checker subprocess, which should implement the grader API
3. Submission subprocess, which runs the submitted code

We can read the outputs of the submission as soon as it flushes to stdout:

for line in p.stdout:
    out += line