* new module: openapi-generator-core
* templating engine adapters to support extension (currently only Handlebars)
* new `-e` templating engine CLI option
* adapt Generator to process Template with the passed TemplatingEngineAdpater
* add a MustacheEngineAdapter to the codegen in the unit tests
* force default MustacheEngineAdapter
* copy new core module in the root Dockerfile
* add processTemplatingEngine to CodegenConfig, to be overriden by Codegen classes if needed
* support multiple file extensions per templating engine adapter
* Extends handlebars experimental adapter with explicit contextual resolvers (e.g. map processing)
* Add new openapi-generator-core/pom.xml to release_version_update.sh
* A detailed message will be logged on missing handlebars helper
* Adds README documentation around template default and beta options
* Moves mustache package under new templating package
* Include built-in handlebars helpers which require explicit registration, and custom `startsWith` helper.
This update allows the root Dockerfile to be used as a development
container and updates run-in-docker.sh to use the same entrypoint script
while maintaining backward compatibility for anyone who has scripted
mappings to /gen and /root/.m2/repository.
