Add support for deserializing fields in addition to properties

[thisisthedave]

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I made changes so that I can deserialize yaml to fields, which should address #117 "Serializing also fields and not just properties will make life easier". This is convenient for Unity objects with [SerializeField] members because Unity disallows using properties for these fields.

I'm not setup to test on platforms other than net46, so there may be issues on other platforms. I added a simple unit test. I'll attempt to execute the docker-based build/test soon when I have a chance to review how it works. I haven't done a pull request on a public repo before, so let me know if there is protocol I'm overlooking.

Thank you for publishing this library! It has been very useful to us.

[ghost]

@aaubry, could this be considered for the next version of YamlDotNet?

[aaubry]

Apparently this PR got lost somehow and in the meantime serialization of fields was independently added to release 5.4.0. Can this be closed, or is there a feature here that is not provided by the current implementation ?