Add package option for mimicking encoding/json field name case handling

[parsley42]

I read the other pull request for similar functionality, but didn't want to implement as part of a Decoder/Encoder because it wouldn't give the behavior I wanted when using standard Marshal/Unmarshal. This non-breaking change adds a package option to preserve the case of
field names when marshaling/unmarshaling.

If this approach is acceptable, I'll be glad to get a launchpad ID and fill out the form, but didn't want to bother without getting feedback first. Thanks!

[rogpeppe]

I'm afraid I don't think that having a global variable that modifies YAML unmarshaling behaviour is a good idea. It means that if you have several packages that use YAML in different ways, they can step on each others' toes. In general, package-level mutable variables should be considered a bad idea.