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.
You should not look if it exists, you should just try to read it and then catch the error there, now your are essential reading the dir twice.