add logstashUdp with webpack support

[alon6699]

when using @log4js-node/logstashudp as an appender it fails when compiling with webpack. (it can't find this module (via require) in runtime). Instead of trying to fix @log4js-node/logstashudp lib, I decided to create a branch that has udp appender and support webpack.

Should I add an explanation inside readme about this appender?