Custom tag support

[beanaroo]

Thanks for this utility! It is great for adding a quick and easy YAML lint to a CI pipeline.

It would be awesome to somehow specify permitted custom tags.

For example, GitLab has the !reference tag that results in:

[error] unknown tag !<!reference> (19:34)
 16 |     - job: node:deps
 17 |       optional: false
 18 |   script:
 19 |     - !reference [.build, script]
---------------------------------------^

AWS Cloudformation is another example and has a bunch of them.

I had a look at the sample code for custom types in js-yaml and do not have any bright ideas on how this could easily be implemented in CLI form:

https://github.com/nodeca/js-yaml/blob/0d3ca7a27b03a6c974790a30a89e456007d62976/examples/custom_types.js#L80