Support YAML anchors/aliases on deserialization

[Blacksmoke16]

With the implementation of #18, YAML input is now streamed, or read token by token as it comes in. This makes supporting aliases/anchors a bit tricky as there isn't way to "back up" the pull parser when an alias is encountered.

Currently there are two way I can see to fix this.

1. Try to come up with something to keep track of/store the anchored elements and apply them when encountered again.
2. Revert Yaml.deserialize to simply use the previous logic of loading it into memory to do the conversion.

1 would be most ideal, but 2 wouldn't be the end of the world.

[Blacksmoke16]

After giving this some thought, I didn't really come up with any ideas for handling it in a streaming fashion. Plan for now is to replace the deserialize method with:

# OPTIMIZE: Figure out a way to handle aliases/anchors while streaming.
def self.deserialize(input : IO, output : IO, **args) : Nil
  YAML.parse(input).to_json output
end

PRs are welcome if anyone can come up with a solution that would allow for streaming as well.