Environment variables instead of CLI options

[alexv-ds]

Added the ability for options to take data from environment variables if the option is not specified in the CLI.

Example usage (docker compose):

service:
  my_app:
    image: myimage
    command: node main.js | npx pino-gelf log
    environment:
      PINO_GELF_HOST: log-server.host
      PINO_GELF_PROTOCOL: tcp
      PINO_GELF_RECONNECTION_DELAY: 2000
      PINO_GELF_KEEP_ALIVE: false

Property	Environment Variable	Default
Host	PINO_GELF_HOST	127.0.0.1
Port	PINO_GELF_PORT	12201
Protocol	PINO_GELF_PROTOCOL	udp
Maximum Chunk Size	PINO_GELF_MAX_CHUNK_SIZE	1420
Keep Alive	PINO_GELF_KEEP_ALIVE	true
Reconnection limit	PINO_GELF_RECONNECTION_LIMIT	-1 (no limit)
Reconnection delay	PINO_GELF_RECONNECTION_DELAY	1000
Verbose Logging	-	false
Passthrough	-	false

[mcollina]

lgtm

[alexv-ds]

Rebased master onto env-options branch
#257