Provide a standard middleware to set c.Env. Don't include it in the
default stack, however, since the RequestID middleware will end up
allocating Env anyways.

Fixes #11

They're really verbose. I should probably DRY them up later.

httptest was adding an extra flag, which was sort of ugly. Instead,
reimplement the parts of its functionality we were using. Bonus: due to
specialization, it's now a bit more efficient as well!

Who knew! (RFC 2616, 5.1.1)

For sendfile(2) support. I should really DRY up the implementations here...

In order to avoid a dependency on the go.crypto terminal package, let's try to
do our own TTY sniffing. I think in practice this will work surprisingly well,
even if it feels incredibly sketchy.

I've currently written three:
- Request ID generation
- Request logging, with color (!)
- Recovery from panics