Refactoring: Yaml parser allows to include external files

Loading of Yaml files allows defining file include markers that load the content of an external file and inserts at the marker's location. This also enables the reuse of aliases defined in the included content.

* a compiler loads Yaml files, assembles it and uses the result for uploading as an ES template
* the `MappingsReader` class loads all types, types that should not appear in the ES template begin with an underscore, e.g. `_default.yml`
* removed Helpers file

Half of the commits resulted in the extraction of functions into a separate gem (see [psych-inherit-file](https://github.com/Asquera/psych-inherit-file)), used to load Yaml files with file inclusions.