allow loading macros globally

I understand the reasoning for declaring macros in every file (explicitness), but if I have 3 macros/packages from npm, and I want to use them in many files, it's a lot of cruft that I want to eliminate.  Especially when it's just converting future syntax to current.

This lets you declare what macros to include when browserifying, and still allows the `import macros from` to work.

The usage in package.json:

```js
"browserify": {
  "transform": [
    ["sweetify", {"modules": ["es6-macros", "sweet-es7-promise"]}]
  ]
}
```

JS API:

```js
.transform('sweetify', {modules: ['es6-macros', 'sweet-es7-promise']})
```