Allow negative marks for a question

[prathamesh920]

Moderator can set negative marks for a question.
This handles the partial grading situation as well.

Note: This is a basic negative marking feature.
This completely depends on the question attribute negative marks
Negative marks setting from Quiz is not included here(can be done as a separate PR if needed).

closes #333

[pep8speaks]

Hello @prathamesh920! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file yaksh/models.py:

Line 1749:80: E501 line too long (80 > 79 characters)

[codecov-io]

Codecov Report

Merging #706 (f98ad48) into master (4756f86) will increase coverage by 4.58%.
The diff coverage is 85.71%.

❗ Current head f98ad48 differs from pull request most recent head 72e7b76. Consider uploading reports for the commit 72e7b76 to get more accurate results

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
+ Coverage   76.69%   81.28%   +4.58%     
==========================================
  Files          53       40      -13     
  Lines        7684     5732    -1952     
  Branches     1166      884     -282     
==========================================
- Hits         5893     4659    -1234     
+ Misses       1408      741     -667     
+ Partials      383      332      -51     

Impacted Files	Coverage Δ
yaksh/models.py	89.23% <80.00%> (+4.06%)	⬆️
yaksh/views.py	71.96% <100.00%> (+1.35%)	⬆️
api/views.py	75.83% <0.00%> (-0.91%)	⬇️
yaksh/bash_stdio_evaluator.py	80.55% <0.00%> (-0.53%)	⬇️
yaksh/cpp_code_evaluator.py	83.67% <0.00%> (-0.49%)	⬇️
yaksh/java_code_evaluator.py	84.00% <0.00%> (-0.47%)	⬇️
yaksh/hook_evaluator.py	78.43% <0.00%> (-0.42%)	⬇️
yaksh/r_code_evaluator.py	79.41% <0.00%> (-0.30%)	⬇️
yaksh/cpp_stdio_evaluator.py	82.35% <0.00%> (-0.26%)	⬇️
yaksh/grader.py	91.34% <0.00%> (-0.25%)	⬇️
... and 24 more

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