Settings to allow customTags and other customizations

[DeltaSigma130]

Summary

Add support to define custom (or local) tags.

Motivation

Allow cleanly parsed YAML files that are not corrupted with custom tags errors.

Describe alternatives you've considered

Local tags are allowed in YAML where the constructors are defined separately from the parser. These local tags generate unknown tag errors.

Additional context

A similar request is found here for an Atom YAML extension, but the parser option seems better suited for this package.

liuderchi/ide-yaml#29