Roadmap 1.0

1. Respond multiple content-types per coproduct endpoint (see #575)
2. Content-type negotiation based on Accept header (see #510)
3. Accept multiple content-types per body* endpoint (see #562 and #335)
4. finch-generic, a standalone package providing generic derivation (see #528)
5. JSON streaming request (see #336 and #557)
6. Figure out whether or not to keep ValidationRule (see #513)
7. Better API for io.finch.items (see #529)
8. Decode JSON directly from Buf (see #511)
9. Better story for Encode.*[Exception] (see #547)
10. Reduce the performance gap between Finagle/Finch to 10% (see https://github.com/finagle/finch#performance)