Add support for pwnwiki.github.io

[grutz]

(Pwnwiki)[https://github.com/pwnwiki/pwnwiki.github.io] is a great static wiki. Kvasir should easily be able to serve up the files given a path in the kvasir.yaml config file.

[grutz]

Since pwnwiki is a totally outside/third-party static HTML environment there are some risks to consider.. What if somebody pwns the pwnwiki? It's risky to run this inside the same dom as Kvasir/web2py so a recommended path:

• set pwnwiki url in kvasir.yaml.sample to the primary wiki URL
• use a warning page to say you're about to visit an untrusted site and to use private mode
• allow user to not show the warning for further views during the session

This is a good thing for all external URLs.