make_pr: Add regenerate-fixtures feature

[unclenachoduh]

• Add regenerate-fixtures feature
• Document in README
• Minor changes to test

[coveralls]

Pull Request Test Coverage Report for Build 102

• 3 of 6 (50.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 90.323%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
make_pluralrules/src/main.rs	0	3	0.0%
<!--	Total:	3	6	50.0%	-->

Files with Coverage Reduction	New Missed Lines	%
make_pluralrules/src/main.rs	1	0.0%
<!--	Total:	1		-->

Totals
Change from base Build 98:	-0.3%
Covered Lines:	532
Relevant Lines:	589

---

💛 - Coveralls