JCasC templating and inheritance support

Currently JCasC does not support YAML templating or inclusion in any means. For example, in https://github.com/oleg-nenashev/demo-jenkins-config-as-code/blob/master/init_scripts/src/main/groovy/scripts/Docker.groovy I create multiple Docker Cloud templates for different agent types. The configuration would be huge and difficult to maintain in JCasC.

I would suggest having an engine similar to what https://github.com/probot/probot-config does using [Deepmerge](https://www.npmjs.com/package/deepmerge):

* Independent YAML files can be created for configuration snippets and templates
* External configuration files can be referenced via `_extends` notion at any level. Local files or other configuration providers (once implemented) can be referenced
* JCasC reads references when loading or validating configs