Support custom headers

[laurencelundblade]

Be able to add arbitrary headers when encoding and be able to call out to custom decoder when encountering an unknown header.

[letmaik]

I'm interested in custom headers specifically for decoding. How much would it take to support this?

[letmaik]

@laurencelundblade How much of this is implemented in the develop branch already? Is it just the string label support that's missing?
cc @plietar

[laurencelundblade]

Custom headers is largely complete and in good shape in the dev branch. I'm happy with the design and implementation.

It does not support string labels, but adding that should be straight forward.

But, what's there is possibly not the path for counter signatures because the call backs don't have access to the signer inputs. I think a new type of signer object is the way to go for counter sigs.

The signer objects are also in fairly good shape except for the verification of multiple signatures. I'm kind of working on that now (in parallel with a lot of other stuff).

[laurencelundblade]

Fixed by #78 and other PRs