using envify purge option in package.json browserify field

It took me a while to get the envify purge option right inside the package.json.

I'm just posting it here for reference.

``` json
"browserify": {
"transform": [
["brfs"],
["envify", {"_": "purge"}]
]
},
```

note: brfs is not needed, it's just showing how to add several transforms.