Adding support for global installs

This PR enables the library to be pushed to NPM, and after that globally installing it.

After, you can simply do `ZABBIX_USERNAME=user ZABBIX_PASSWORD=password ZABBIX_JSON_API_URL=https://myzabbix.domain.tld/api_jsonrpc.php PORT=8888 zabbix-monitor`

And the user do not need to install deps manually, cloning from git, etc...

PS: Ooops, sorry that I forgot to create a feature branch, totally missed that part :/