Add -D flag to print timestamp in front of output lines.

I'm looking to support fping as an ICMP measurement tool in the netperf-wrappers utility for testing bufferbloat (see https://github.com/tohojo/netperf-wrappers). In order to do this, I need to be able to timestamp output, to correlate it with bandwidth measurements.

This pull request contains a patch to add a -D flag which will print a UNIX timestamp before each output line, similar to how the Linux `ping` utility works.