Adding Cake YAML Support for YAML Merge

[DaveDiehl]

I needed the ability to parse a YAML file with a merge key. The YAML DotNet framework supports it (see this StackOverflow article), but the parser used before will not support it.

To do this, you need to use the MergingParser in place of the StringReader.

 using (var tr = new StringReader(yaml))
        {
            var reader = new EventReader(new MergingParser(new Parser(tr)));
            result = d.Deserialize<T>(reader);
        }

I added tests to make sure this functionality works and is backward compatible.

[critesjm]

Hey @Redth, would you mind giving this a peek?

[Redth]

lgtm! 🚢