perl: New module implementing a Perl destination

This new module implements a fairly simple Perl destination, with which
one can write destination modules in Perl. The module will call three
functions: init, deinit and queue (the name of all three are
configurable, of course). The first two receive no arguments, the queue
method receives a hash map of key-value pairs, selected by
value-pairs().

It does some type hinting (int and string supported only, so far), and
does not support sub-hashes yet.

This fixes #21.

Requested-by: Fabien Wernli
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>