(yaml) Redesign StringQuotingChecker added in 2.12

[cowtowncoder]

Looks like I severely under-designed StringQuotingChecker so that:

1. Handler is not passed enough pertinent information about either output context or generator configuration
2. Handler can only give "yes"/"no" style decision on whether to use "double quotes" or something else (likely "plain"); but underlying SnakeYAML has up to 5 options (double/single-quoted, literal, folder, plan)

It would be better to solve these, and although this may be tricky to implement with full backwards-compatibility, I think it can in practice be done without problems for 2.13 -- initial implementation is so limiting that it probably isn't very useful to anyone as-is.