Type boolean not supported

[jsshapiro]

yaml-validator does not currently provide a way to indicate that a key should have a boolean value. This seems like something that could easily be added.

Note that the acceptable boolean values changed between YAML 1.1 and YAML 1.2 in a way that was not backwards compatible. I'm not sure whether js-yaml makes it possible to specify which version should be parsed, but if it does, I think it would be helpful to be able to specify that in the yaml-validator options structure.

[paazmaya]

Would you be willing to try that in a PR?