Add new-file-notification via NATS

[digizeph]

This PR adds new-file notification by publishing newly indexed BGP MRT file meta information in JSON format to user-specified NATS server.

The NATS URL and credentials is configured via environment variables:

• BGPKIT_BROKER_NATS_URL like nats://nats.bgpkit.com:4222 (default)
• BGPKIT_BROKER_NATS_ROOT_SUBJECT such as public.broker. (default)

The notification is published to public.broker.{PROJECT}.{COLLECTOR}.{DATA_TYPE} where

• PROJECT is riperis or route-views
• COLLECTOR is the route collector IDs like rrc00 or route-views2
• DATA_TYPE is rib or updates

Example of what a subscriber can be notified for:

• public.broker.>: any new MRT files
• public.broker.route-views.>: new RouteViews updates files
• public.broker.rrc00.rib: new RIPE RIS rrc00 RIB dump files
• public.broker.*.rib: any new RIB dump files

This PR also adds a new subcommand bgpkit-broker live that listens to the specified NATS server for new file notification.

Streaming live from a broker NATS server

Usage: bgpkit-broker live [OPTIONS]

Options:
      --no-log             disable logging
  -u, --url <URL>          URL to NATS server, e.g. nats://localhost:4222. If not specified, will try to read from BGPKIT_BROKER_NATS_URL env variable
      --env <ENV>          
  -s, --subject <SUBJECT>  Subject to subscribe to, default to public.broker.>
  -p, --pretty             Pretty print JSON output
  -h, --help               Print help
  -V, --version            Print version

This resolves issue #41.