Feature Request: Set source_id and api_key in env

[ttilberg]

I was looking at the code that configures the source/api_key in the HTTP log device, and was wondering if there was a specific reason the API key and Source do not have an env configuration as well. If there isn't one, can we add it?

        @api_key = api_key || raise(ArgumentError.new("The api_key parameter cannot be blank"))
        @source_id = args.first
        @timber_host = options[:timber_host] || ENV['TIMBER_HOST'] || TIMBER_HOST
        @timber_port = options[:timber_port] || ENV['TIMBER_PORT'] || TIMBER_PORT