Environment variables instead of CLI options

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`

pinojs / pino-gelf

Environment variables instead of CLI options #256

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