CMake: Prefix options with "YAML" and hide platform-specific options (#734)

* CMake: Prefix options with "YAML" and hide platform-specific options

When including yaml-cpp as a subproject, some option names can conflict with other projects.

(1) Make sure the yaml-cpp options are prefixed with YAML
(2) Hide platform-specific options when possible to avoid cluttering the cmake option list

* Update docs for change from BUILD_SHARED_LIBS to YAML_BUILD_SHARED_LIBS