Allow configuring the way "must quote" is determined for property names, String values

[cowtowncoder]

(note: inspired by #226 and #68 before that)

Since there are different preferences on quoting various keys, String values, it may not be possible to find perfect set that works for everyone. So let's allow configuring of the set; YAMLMapper.builder() should make it possible.

[cowtowncoder]

Decided to make things bit more configurable by introducing StringQuotingChecker abstraction, configurable via YAMLFactory.builder(); base implementation StringQuotingChecker.Default.