Add --autoreload flag

[afshin]

Port of jupyter/notebook#4795. Fixes jupyter/nbclassic#39

[codecov-io]

Codecov Report

Merging #380 (84ca636) into master (4cf7d4d) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   69.07%   69.15%   +0.07%     
==========================================
  Files          56       56              
  Lines        6221     6224       +3     
  Branches      816      816              
==========================================
+ Hits         4297     4304       +7     
+ Misses       1650     1648       -2     
+ Partials      274      272       -2     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	60.16% <100.00%> (+0.13%)	⬆️
jupyter_server/services/kernels/handlers.py	58.51% <0.00%> (+0.26%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	81.42% <0.00%> (+1.18%)	⬆️

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[kevin-bates]

Thanks @afshin - confirmed the changes match those of the referenced NB PR sans additional formatting.