#1931 YAML Lexer has incorrect style for certain comments

[Jim Pattee]

All platforms, version 3.7.4.
The YAML lexer misses styling comments in certain instances.
The attached file, appveyor.yaml, shows the error on lines 10, 57-58, and 61-64.
The attached LexYaml.cxx will fix the problem.

The attached LexYAML.cxx also styles the optional "..." at the end of file.
It uses the same style as the "---" at the begining of the file.

[Neil Hodgson]

Thanks, committed as [4e2b6c].