Add host address env var override and update README

[vicchi]

---

Here's the reason for this change :rocket:

• Added support for overriding the default network address to bind to via the HOST environment variable, in line with (almost) all of the other Pelias services.

  Here's what actually got changed :clap:

• Update index.js adding support for the HOST env var and passing this to app.listen()

• Small update to README.md documenting this

---

Here's how others can test the changes :eyes:

Without HOST defined ...

PORT=4100 npm start
<snip>
2020-07-16T15:29:14.509Z - info: [placeholder] [worker 92474] listening on 0.0.0.0:4100

sudo lsof -i -P -n | grep LISTEN | grep 4100
node      92601          pelias   36u  IPv6 513692      0t0  TCP *:4100 (LISTEN)

With HOST defined ...

HOST=127.0.0.1 PORT=4100 npm start
<snip>
2020-07-16T15:31:23.397Z - info: [placeholder] [worker 92745] listening on 127.0.0.1:4100

$ sudo lsof -i -P -n | grep LISTEN | grep 4100
node      92737          pelias   36u  IPv4 501663      0t0  TCP 127.0.0.1:4100 (LISTEN)

[orangejulius]

Sweet. I'm sure lots of folks will appreciate having this configurable everywhere!