Add Map-reduce functionality

[zix99]

Add functionality that allows:

• Reducing with an expression for more diverse functionality than just adding
• Add More than one group to reduce against

The intersection between table and analyze.

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[codecov-commenter]

Codecov Report

Merging #87 (54f4776) into master (d2ef5ee) will increase coverage by 0.58%.
The diff coverage is 96.85%.

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@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   87.60%   88.18%   +0.58%     
==========================================
  Files          98      100       +2     
  Lines        4702     5046     +344     
==========================================
+ Hits         4119     4450     +331     
- Misses        446      454       +8     
- Partials      137      142       +5     

Impacted Files	Coverage Δ
cmd/commands.go	100.00% <ø> (ø)
cmd/reduce.go	92.94% <92.94%> (ø)
cmd/analyze.go	96.38% <100.00%> (+0.04%)	⬆️
cmd/bargraph.go	100.00% <100.00%> (ø)
cmd/docs.go	100.00% <100.00%> (ø)
cmd/expressions.go	92.37% <100.00%> (+0.06%)	⬆️
cmd/filter.go	85.93% <100.00%> (+0.22%)	⬆️
cmd/heatmap.go	96.47% <100.00%> (+0.04%)	⬆️
cmd/histo.go	91.08% <100.00%> (+0.08%)	⬆️
cmd/tabulate.go	99.19% <100.00%> (+<0.01%)	⬆️
... and 2 more

... and 1 file with indirect coverage changes

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