Improve parsing of plain (unquoted) scalars

[biojppm]

Fixes #60

[coveralls]

Pull Request Test Coverage Report for Build 457

• 162 of 175 (92.57%) changed or added relevant lines in 4 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.1%) to 92.737%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/c4/yml/common.hpp	8	10	80.0%
src/c4/yml/common.cpp	13	17	76.47%
src/c4/yml/parse.cpp	140	147	95.24%

Files with Coverage Reduction	New Missed Lines	%
src/c4/yml/common.cpp	1	83.67%
src/c4/yml/common.hpp	1	89.19%
src/c4/yml/parse.cpp	5	89.76%

Totals
Change from base Build 449:	0.1%
Covered Lines:	3422
Relevant Lines:	3690

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💛 - Coveralls

[codecov]

Codecov Report

Merging #61 into master will increase coverage by 0.15%.
The diff coverage is 93.99%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   93.13%   93.28%   +0.15%     
==========================================
  Files          58       58              
  Lines        5548     5676     +128     
==========================================
+ Hits         5167     5295     +128     
  Misses        381      381              

Impacted Files	Coverage Δ
test/double_quoted.cpp	100.00% <ø> (ø)
test/plain_scalar.cpp	100.00% <ø> (ø)
test/simple_doc.cpp	100.00% <ø> (ø)
test/single_quoted.cpp	100.00% <ø> (ø)
src/c4/yml/common.cpp	81.39% <75.00%> (+9.96%)	⬆️
src/c4/yml/common.hpp	88.57% <80.00%> (+1.90%)	⬆️
src/c4/yml/parse.cpp	89.50% <95.20%> (+0.36%)	⬆️
test/test_case.cpp	76.14% <96.29%> (+1.22%)	⬆️
src/c4/yml/parse.hpp	100.00% <100.00%> (ø)
test/github_issues.cpp	97.24% <100.00%> (+0.99%)	⬆️
... and 4 more

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