add support for Collaborator

[dnet]

Some RCE tests would work much more reliably using Burp Collaborator:

• [x] PhpPregArray used phpcredits()
• [ ] RoRCheck could also be augmented with this, it currently injects id and looks for uid= and gid= in the response
• [ ] same with CodeExec which currently uses sleep and ping and measures response time

In both cases above, nslookup could be used to interact with Collaborator in a platform-independent way:

nslookup [is] available on both windows and *nix systems by default. Ping is an obvious alternative, but when invoked on Linux it never exits and thus may hang the executing thread.