Copy sandbox options when recursing

[OliverJAsh]

See #41.

Given:

var SandboxedModule = require('sandboxed-module');

SandboxedModule.require('./foo', { sourceTransformers: {
    test: function (source) {
        console.log('test source transformer:', this.filename);
        return source;
    }
}});

I would expect that any calls to require inside of the sandboxed module would carry the same options (e.g. sourceTransformers) as the initial sandboxed module.

[domenic]

Can you add a test?