Implement support for HTTP trailers, upstream to Plug

[mtrudel]

As discussed at https://github.com/elixir-plug/plug/pull/535#issuecomment-555781624, Plug lacks suppor for HTTP trailers. Beyond their usefulness in the real world, they could also help to tie off a couple of awkward spots when dealing with chunk encoding in HTTP/1 (as discussed on the linked thread above).

At least in h2, we already implement support for reading trailers, but currently just log and discard them.