Use ABNF.

[ioggstream]

This WIP PR

Use ABNF to define headers values.

Todo:

• [x] reference ABNF rfc5234
• [x] define the representation-data-digest as digest-algoritm = value

Further considerations

How to pass digest-algoritm parameters? eg.

1. pass parameter to an algoritm

sha-512; trim=256=fafafafa,  sha-256=babababa 

2. pass parameter to value

   sha-512=fafafa; trim=fafafafa, sha-256=babababa

[LPardue]

So if we're going to use the #rule extension for ABNF, its probably worthwhile adding this into the notational conventions like I do https://tools.ietf.org/html/draft-pardue-quic-http-mcast-04#section-1.1

e.g.

   This document uses the Augmented BNF defined in [RFC5234] and updated
   by [RFC7405] along with the "#rule" extension defined in Section 7 of
   [RFC7230]. 

we could also stick anything else we need there too.