Update Docs + API - matchingFunc

[jbenet]

Update Docs + API after the matchingFunc discussion. We discussed:

• multistream takes in a tuple: (multicodec-header, handlerFunc, matchingFunc)
• handlerFunc is called with the header
• matchingFunc decides whether a header matches.
• default matchingFunc is equivalence (tests the given header matches the original one)

// something like this
type matchingFunc func(h multicodecHeader) bool
type handlerFunc func(h multicodecHeader, s io.Stream)