add ip_lookup option to initializer template

I noticed this option was in the README, but not in the template, so it
might be nice to have it generated in the template as well.

lib/generators/geocoder/config/templates/initializer.rb

@@ -2,6 +2,7 @@ Geocoder.configure(
   # Geocoding options
   # timeout: 3,                 # geocoding service timeout (secs)
   # lookup: :google,            # name of geocoding service (symbol)
+  # ip_lookup: :freegeoip,      # name of IP address geocoding service (symbol)
   # language: :en,              # ISO-639 language code
   # use_https: false,           # use HTTPS for lookup requests? (if supported)
   # http_proxy: nil,            # HTTP proxy server (user:pass@host:port)