Add option to configure PROXY for the system

[derneuere]

If a system is behind a proxy, then the "npm install" commands does not honour the proxy global proxy settings on the machine, so it can't reach the https://registry.npmjs.org. I suggest that the proxy be set as optional variables at the top of install-librephotos.sh and then used in commands something like below just before the 'npm install' commands if PROXY is not blank.

npm config set proxy http://$PROXY:$PROXY_PORT
npm config set https-proxy http://$PROXY:$PROXY_PORT