Add global macros array for node loader.

Created by: Paxa

For my opened issue #294 (closed):

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:

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

sweet.loadMacro('./macros/puts');
puts 123; // not working