Macro support

This pull request introduces the concept of *macro* (maybe it can be named something more meaningful).

Applications built with harp can include a `_macros` folder in the project root and add node.js modules into this folder. Functions and objects exported by modules will be available in the global scope and accessible from templates as

```html
<%- macros.someFunction() %>
```

This helps keeping code/logic out of templates and avoid code like http://harpjs.com/recipes/blog-sitemap.

It opens up harp to lots of freedom for more expert developers who want a more customized harp experience.