Add retrieve submission-summaries command

[qryxip]

bors r+

[codecov-commenter]

Codecov Report

Merging #16 into master will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master     #16      +/-   ##
=========================================
- Coverage    1.54%   1.50%   -0.05%     
=========================================
  Files          20      21       +1     
  Lines        2335    2399      +64     
  Branches     1124    1154      +30     
=========================================
  Hits           36      36              
- Misses       2239    2309      +70     
+ Partials       60      54       -6     

Impacted Files	Coverage Δ
src/commands/retrieve_submission_summaries.rs	0.00% <0.00%> (ø)
src/lib.rs	0.74% <0.00%> (-0.01%)	⬇️
src/shell.rs	0.00% <0.00%> (ø)
src/web/mod.rs	0.00% <0.00%> (ø)

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[bors]

Build succeeded:

• grcov
• Rustfmt
• stable-x86_64-apple-darwin
• stable-x86_64-pc-windows-msvc
• stable-x86_64-unknown-linux-gnu