Support KDL as a configuration format

Hugo currently supports TOML, YAML, and JSON configuration files, and I propose adding KDL as another supported configuration format. For example, Hugo could recognize a hugo.kdl file.

This would be useful for projects that already use KDL, and these projects could use the same configuration syntax across their tools.

The implementation could recognize hugo.kdl as a Hugo configuration file and convert the KDL document to Hugo's existing configuration structure.