Feature: Allow YAML multiple documents

[bbompk]

This feature allows YAML validator to be able to parse multiple documents.

Currently came up with a simple solution: if fails to parse a file parse which allows only single document YAML, will try to parse it as multiple documents if allow_multiple_documents flag is true, and then try to parse each document again.

Have plenty of room for features

• validate with schema ?
  • how strict ?
• more flags or options about multi document parsing
• more info logged from getting error when parsing

p.s. will need assist on the process of building for release

[bbompk]

@GrantBirki Please kindly review PR, thank you

Since last change: Added parsing errors to validation report (or should I move it to log with core.error ?)

If approved, I would like to also kindly ask for a guideline on building for release as well (I apologise for I don't know how, sorry :sweat_smile: )