Add "Context" constructor

[pluma]

Digging through the prototype chain of the result of node's vm.createContext reveals that objects returned by it are instances of Context, which seems to be a simple constructor function with a prototype that is a plain old object.

So to emulate node's behaviour, the following needs to be added:

function Context() {}
Context.prototype = {};

• createContext should return a new Context rather than a new object. It should still perform a shallow copy.
• runInContext should throw a TypeError if context is not an instance of Context (having the right prototype is not sufficient). The code that is currently in runInNewContext should be moved here.
• runInNewContext should call runInContext with the context argument passed through createContext.

I can create a PR for this behaviour if you want.