Provide offical config schema

[Unique-Pixels]

IDEs provide code completion when schemas are provided for yaml files. It would be helpful to have official schemas. I found this: https://github.com/yu-iskw/allstar-jsonschema however the project should provide official schema to ensure compliance.

[wesley-dean-flexion]

I, too, would like to see schemae provided for the configuration files, possibly shared with the JSON Schema Store (which accepts YAML, despite the name). Not only would this allow supported editors and IDEs to hint at proper usage, but it would allow tools such as @chris48s's v8r -- which is included as one of the linters in MegaLinter -- to validate configuration files. This may help with issue such as those in #158 where it can be challenging to acquire feedback to debug invalid configurations.