trigger example for plsample

Hi everyone,

I've adapted the work that Konstantina did for pl/julia as part of her
GSOC project to add an example of handling triggers to plsample.  Which
was based from pl/tcl and pl/perl.

One aspect that I'm not sure about is whether the example should be
duplicating code (as it is now) for keeping an example contained within
a single function.  The only reason I can come up with is to try to read
through an example with minimal jumping around.

Hoping this is a good start.

Regards,
Mark