Enable support for tags

[EliSnow]

It would be nice if YAML tags could be used as a representation for an enum tag representation.

I don't have it fully fleshed out since I'm not sure how the internals of serde parsing works, but perhaps something like:

foo: !bar baz

could automatically be interpreted as:

foo:
  bar: baz

representing an enum type which is externally tagged. Again I don't know how serde internals work, but could something like this work with any enum representation?

Could be related to #91

[dtolnay]

Thanks, I would be willing to consider a PR that implements this.