Allow global transforms

This patch adds a corresponding `globalTransforms` option that behaves just like the `transforms` option, except `{global: true}` is passed as the first argument to `bundle.transform()`

I'm not married to the implementation, but it seemed like the least intrusive both in exposed interface and required code.

It needs tests, which I'll write once we're sure it's a desired feature.

_(I want this functionality for [uglifyify](https://github.com/hughsk/uglifyify#global-transforms), but it's a browserify option that's generally useful)_