Adds support for CSP-strict environments like node-webkit and atom-shell

I'm writing a rather complicated package for the Atom editor and I require templating functionality. In order to avoid running my server/templating in a node subprocess, I've added a `safeMode` flag to swig which prompts swig to use [`loophole`](https://github.com/atom/loophole) to run the compilation in a vm context. This allows my Atom package to be installed/run normally. Without this functionality swig cannot be used as a dependency in environments like `node-webkit` or `atom-shell` that have (rightfully so) strict content security policies.

Thanks in advance for your consideration.

-Joe