Implementation for Merge Key Language-Independent Type for YAML™ Version 1.1

[T0ha]

This merge request adds implementation for Merge Key Language-Independent Type for YAML™ Version 1.1 into yamerl for YAML 1.1 schema.

We also added merge functionality for list type even though there is no one in the spec since we needed it for our project.

[dumbbell]

Awesome! Thank you for the feature :)

Could you please add a testsuite as well?

[T0ha]

Ok. I'll do in meantime.