The `/redirect-to` endpoint currently acts as an open redirect, which is
bad for any go-httpbin instance exposed to the public internet.

This allows configuring an allowlist of domains to which traffic can be
redirected.

This adds a new /hostname endpoint as originally proposed in #66. In
this implementation, it exposes a dummy hostname by default, and only
exposes the real hostname (via `os.Hostname()`) if the
`-use-real-hostname` flag is given on the command line.

In #17, we switched to deploying httpbingo.org on Google App Engine. To
meet their naming requirements, we had to rename cmd/go-httpbin to
cmd/go_httpbin, but this broke any existing uses of, e.g.,

    go get github.com/mccutchen/go-httpbin/cmd/go-httpbin

as suggested in the README and as used in various places (including my
employer).

Here's a dumb ass fix for that dumb ass problem; I'm not happy about it,
but it seems to work.

Add support for an observer that will be called with the result of each
request handled, which can be used to log or otherwise instrument
the service.

This will simplify deployment on some platforms (e.g. heroku)