Support ignoring tags, without needing a schema

[multimeric]

I have a complex data model that uses tags, but at the point where js-yaml touches it, I really don't care about the tags. I would like to be able to load the data without manually specifying each and every custom tag I've used as a schema.

Might it be possible to have a PERMISSIVE_SCHEMA that simply takes mapping types and converts them to JS objects, and takes sequence types and converts them to arrays? I suspect this is a somewhat common use-case.

In general, this would be a quick-and-dirty solution to the error:

YAMLException: unknown tag !<!MyTag> at line 8, column 1:

[puzrin]

Dupe of #385

[rlidwka]

You can now construct custom permissive schema, feature added in 73ef02c (currently in dev branch, not released yet).

(example: https://github.com/nodeca/js-yaml/blob/a0d0caa5aa0f5354fefa9c637cfb7c4c17ef7d02/examples/handle_unknown_types.js)