Add support for mocks in tables

[JosiahWitt]

Closes #6

TODO:

• [x] Add support for filling Mocks field in tables (#17)
• [x] Add support for running SetupMocks field in tables (#18)
• [x] Add support for filling the subject using interface checks (#19)
• [x] Build wrapper around mockgen to generate multiple mocks and add NEW method (concurrent generation as a follow up) (#22)
• [x] Address comments (#24)
• [x] Update docs (#21)

[codecov-io]

Codecov Report

Merging #17 (18943a5) into master (30a5844) will increase coverage by 1.15%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   98.19%   99.34%   +1.15%     
==========================================
  Files           5       11       +6     
  Lines         166      461     +295     
==========================================
+ Hits          163      458     +295     
  Misses          2        2              
  Partials        1        1              

Impacted Files	Coverage Δ
cmd/ensure/internal/cmd/generate_mocks.go	100.00% <100.00%> (ø)
cmd/ensure/internal/cmd/root.go	100.00% <100.00%> (ø)
cmd/ensure/internal/ensurefile/ensurefile.go	100.00% <100.00%> (ø)
cmd/ensure/internal/mockgen/mock_destination.go	100.00% <100.00%> (ø)
cmd/ensure/internal/mockgen/mockgen.go	100.00% <100.00%> (ø)
cmd/ensure/internal/runcmd/runcmd.go	100.00% <100.00%> (ø)
ensurepkg/ensurepkg.go	91.66% <100.00%> (+3.66%)	:arrow_up:
ensurepkg/run_table.go	100.00% <100.00%> (ø)
... and 4 more

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[JosiahWitt]

Consider logging when skipping assigning to fields in subject. That might help when changing an interface and forgetting to regenerate mocks. Maybe the warning should be for unused mocks? Or just when no mocks match a field that is an interface?