Add tc --baseline option and associated logic and documentation

[martinholmer]

This pull request adds a --baseline BASELINE option to the command-line tool tc. Without this option, tc uses current-law policy as the baseline. When using this option, tc uses the reform specified in the BASELINE JSON reform file as the baseline.

The most common use of this new capability (at least in the near future) will probably be to engage in counter-factual history: what if some other reform (other than TCJA) had been enacted in late 2017. Such an analysis would use the new option this way:
--baseline 2017_law.json
and then specify the alternative reform using the --reform option this way:
--reform 2017_law.json+alternative_reform.json

This pull request resolves enhancement-request issue #1770.

[codecov-io]

Codecov Report

Merging #1851 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1851   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3141    3162   +21     
======================================
+ Hits         3141    3162   +21

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

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