Move export options to config file with env read support

[dogancanbakir]

Proposal

Move export related options to config file similar to nuclei's reporting config file with env read support.

   -ec, -export-config string  proxify export configuration file

Following options will be replaced with -export-config option.

EXPORT:
   -max-size int              Max export data size (request/responses will be truncated) (default 9223372036854775807)
   -elastic-address string    elasticsearch address (ip:port)
   -elastic-ssl               enable elasticsearch ssl
   -elastic-ssl-verification  enable elasticsearch ssl verification
   -elastic-username string   elasticsearch username
   -elastic-password string   elasticsearch password
   -elastic-index string      elasticsearch index name (default "proxify")
   -kafka-address string      address of kafka broker (ip:port)
   -kafka-topic string        kafka topic to publish messages on (default "proxify")

Example config file:

kafka:
  # topic is kafka topic name to publish messages
  topic: proxify
  # address is kafka host to connect with in ip:port format
  address: $KAFKA_HOST

elastic
  # index is elastic index name
  index: proxify
  # elastic username
  username: $ELASTIC_USER
  # elastic password
  password: $ELASTIC_PASSWORD
  # elastic address in ip:port format
  address: $ELASTIC_HOST
  # enable / disable use of ssl for elastic host
  ssl: true
  # enable / disable ssl verification for elastic host
  ssl-verification: false

Closes #262.