Add after-tax income change by income decile graph to tc --graphs output

[martinholmer]

This pull request adds to tc --graphs output a bar graph that shows the reform-induced percentage change in aftertax expanded income by prereform expanded income deciles, with detail on the top decile (for the bottom half, the next four percent and the top one percent). The graph has been available to those doing Python programming with the taxcalc package, but now it is also available to those using the tc command-line tool.

This pull request resolves issue #1674.

[codecov-io]

Codecov Report

Merging #1758 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1758   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        2993    3000    +7     
======================================
+ Hits         2993    3000    +7

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 914f14c...d8b6cc6. Read the comment docs.