Add tax differences table to tc --tables output

[martinholmer]

Now in addition to the reform totals distributional table, the tc --tables option produces a reform minus current-law-policy tax differences table by expanded-income deciles.

[codecov-io]

Codecov Report

Merging #1265 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1265      +/-   ##
==========================================
+ Coverage   99.68%   99.68%   +<.01%     
==========================================
  Files          38       38              
  Lines        2813     2837      +24     
==========================================
+ Hits         2804     2828      +24     
  Misses          9        9

Impacted Files	Coverage Δ
taxcalc/taxcalcio.py	100% <100%> (ø)	⬆️

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