executors: add basic autodetection from filename

[Xyene]

Ref #967

[codecov-commenter]

Codecov Report

Merging #968 (3e582fb) into master (2871cb3) will decrease coverage by 5.93%.
The diff coverage is 74.19%.

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
- Coverage   86.55%   80.62%   -5.94%     
==========================================
  Files         138      138              
  Lines        4737     4758      +21     
==========================================
- Hits         4100     3836     -264     
- Misses        637      922     +285     

Impacted Files	Coverage Δ
dmoj/executors/__init__.py	72.41% <64.28%> (-7.59%)	⬇️
dmoj/utils/helper_files.py	87.50% <82.35%> (-1.74%)	⬇️
dmoj/control.py	35.29% <0.00%> (-64.71%)	⬇️
dmoj/cptbox/isolate.py	35.29% <0.00%> (-60.51%)	⬇️
dmoj/cptbox/tracer.py	58.99% <0.00%> (-26.26%)	⬇️
dmoj/cptbox/handlers.py	75.00% <0.00%> (-25.00%)	⬇️
dmoj/judge.py	51.57% <0.00%> (-18.87%)	⬇️
dmoj/executors/compiled_executor.py	73.29% <0.00%> (-15.91%)	⬇️
dmoj/utils/os_ext.py	80.00% <0.00%> (-15.00%)	⬇️
dmoj/result.py	82.66% <0.00%> (-8.00%)	⬇️
... and 6 more

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