Add a way to set browserify options

While in "dev" mode I wanted to set `insertGlobals` so that build times were faster (for us it cuts the build time in over half from 1000ms to 400ms). I didn't see anyway to set this from a glance so I added it by allowing atomify-js to accept a `browserify` option hash inside the normal option hash. Like this:

```js
atomifyjs({
  entry: 'foo.js',
  browserify: { insertGlobals: true }
});
```