Require hook (server side node).

It would be cool to have a require hook built into this (similar to babel, coffee-script, etc).
So that we could do:

``` js
require("nunjucksify/register");
var myTemplate = require("./index.nunjucks");
```

Which would return a server side, ready to render, nunjucks function when required.
This is useful for apps that use nunjucks isomorphically.
