Add support for SAML authentication

[jwidavid]

This PR will serve as the base for several other PR's. As they get reviewed and approved, I will merge them into this one once the feature is a functioning MVP.

• Update gem to identify the SAML redirect
• Update gem to add SAML flag to follow the redirect and open the browser (or submit via headless browser)
• Update gem to accept and handle session and cookies
• Clean up and complete authentication and scan

---

Testing Instructions

These specific instructions are dependent on being able to build and run the WPScan CLI Scanner locally (rather than through docker).
See: https://github.com/wpscanteam/wpscan

Update wpscan/Gemfile to add:
gem 'cms_scanner', path: '/absolute-path-to-the-scanner/CMSScanner'

You may also need to comment the following line from the wpscan.gemspec:
s.add_dependency 'cms_scanner', '~> 0.13.9'

• Run the scanner using the new --expect-saml flag
• To test against a server with SAML authentication you can use http://3.135.88.75/

[coveralls]

coverage: 98.84% (-1.1%) from 99.925%
when pulling 9275a62 on add/saml-authentication
into e4f555a on master.

[miguelxpn]

Can't really approve it since it's a draft state, but LGTM, we can probably go forward with the other ones and then merge them into this branch.