Add distractors to parsons problems

[igordsm]

Proposal:

!!! exercise parsons description ˋˋˋpython correct code ˋˋˋ

ˋˋˋpython these lines are distractors ˋˋˋ

@toshikurauchi thoughts on this?

[toshikurauchi]

I think it would be nice to support multiple solutions. How about:

!!! exercise parsons
    Description

    ```python
    Solution 1

    Solution 2

!!! distractor
    ```python
    Distractors
    ```

[igordsm]

It seems reasonable to mark distractors explicitly inside the exercise. I wouldn't implement alternative solutions yet on the same PR as this one. For me these are two different things and I would rather create more parsons problems before committing to multiple solutions.

[toshikurauchi]

Agreed. If we have the explicit marker for distractors the syntax won't break when/if we decide to support alternative solutions.

[rafaelcorsi]

I got distracted reading the issue