Allow a dash(-) at the first character of a plain scalar

[fktn-k]

According to the YAML spec, a dash(-) at the first character of a scalar is allowed. link
However, fkYAML didn't support such cases and threw an exception, e.g., with the following input:

foo: -bar

So, this PR has fixed the issue and the fix has also been tested by the added test cases.

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• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
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Please don't

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[coveralls]

coverage: 100.0%. remained the same when pulling 1be03fdf19bc73f7b4e1f229be0395445d56a8c5 on bugfix/allow_dash_as_1st_plain_scalar_char into dc4e22d61dc2433e31be837476523b5ce413be28 on develop.