Add global macros array for node loader.

For my opened issue #294:

`sweet.loadedMacros` - array of macros used with node-loader.
`sweet.loadMacro(file)` - helper to load macros. Same as `sweet.loadedMacros.push(sweet.loadNodeModule(process.cwd(), './macros/str'))`

Example:
```js
var sweet = require('sweet.js');
sweet.loadMacro('./macros/str');
require('./test.sjs');
```

Note: it doesn't work if we load macros inside executing file and use in same file

```js
sweet.loadMacro('./macros/puts');
puts 123; // not working
```