Add support for CBOR tags

[rklaehn]

Using a thread local value as a way to get the tag value over the fence from the CBOR format to a CBOR aware serializer/deserializer.

Replaces #164

Here is a more complex real world use case from @vmx : https://github.com/vmx/rust-ipld-dag-cbor

[rklaehn]

As discussed in #164, the tags feature should not be enabled by default since it changes the deserialization behavior for non tag aware data structures when tags are present.

Not sure if serde_cbor::Value::Tag should exist when the feature is disabled.

[pyfisch]

Thanks for all your work! 🎉

I will release the new version to crates.io soon.