This is a very small improvement to developer ergonomics.

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.

* Stop testing on tip, it takes too long

* Refactor automatic handling of HEAD requests

* Update README

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