Add support for `Normal` `ColorMode`

[LafeWessel]

When attempting to deserialize a KML that has a colorMode set to normal, I get an error saying Invalid color mode: normal. According to the spec that the documentation cites, the default value for this field is "normal". Thus, the value of "normal" should be deserializable, and I would argue that instead of setting the colorMode to "default", it should be "normal" when serializing KMLs.

[pjsier]

Thanks for catching this! Looks like I missed this on the initial implementation. If you’re interested in submitting a PR feel free, otherwise I should be able to take a look next week