Add a Ronin::Payloads::PerlPayload class

[postmodern]

Add a PerlPayload base class for all perl payloads. It should set encoder_class PerlEncoder and define a .payload_type method which returns :perl. Also update Ronin::Payloads::CLI::Printing to add a perl: 'Perl' mapping.

It must also define a to_command method which wraps the Perl payload in a perl -e "..." command.