Add host address env var override and update README

[vicchi]

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Here's the reason for this change 🚀

• 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.

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Here's what actually got changed 👏

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

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Here's how others can test the changes 👀

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!