add bell output on successful pings

the rationale here is that it's actually pretty hard to do this with a
regular ping. you need a silly shell loop and it doesn't always work
right everywhere, because the output of the system ping is
platform-dependant. it also buffers stdout in some weird ways sometimes.

therefore, i think it's a great addition to oping.

the purpose of this is that it can be useful to "hear" ping packets
come back when doing network diagnostics. obviously, this will be
useless in finding out *failed* hosts if multiple hosts are selected,
as any sucessful host will produce a beep. but it can nevertheless be
used to trace network cables or problems without looking at the
console. i also use audible pings to let me know when a hosts returns
after a reboot.

Note that I had to struggle quite a bit to make my terminal bell work,
the following articles were used to generate documentation on how to
make that work reliably:

https://askubuntu.com/questions/228096/terminal-bell-doesnt-ring

also see the following for the original inspiration for this:

http://catb.org/jargon/html/P/ping.html
https://groups.google.com/forum/#!msg/comp.sys.next/JDaeD8oqarU/v8xaDS8kXM0J