Add specified handlers

[xyzkab]

I want ssrfmap to able to just do quick detection if specified params are likely prone to ssrf attack. This pull request consist:

• Set reverse shell as default handler if no argument specified on -l option
• Move cmd shell execution from core/ssrf.py#L43 to core/handler.py as listen_command method
• Load specified handler from ./handlers directory
• Add handlers/http.py to listen for out of band request from target application
• Add httpcollaborator module to test multiple specified params (like auxiliary module)

This functionality added will look like this

@swisskyrepo what do you think of this approach? Thanks

[swisskyrepo]

Great work, I like this module :) 👍