Use watchify if watch option is true

This works, but for some reason the re-bundle runs twice. Rather than bang my head against the wall, I am hoping somebody else can spot my mistake. In case it's helpful, a simple test setup is below.

```
// main.js
var dep = require('./dep');
```

```
// dep.js
module.exports = 'I am a dep yep';
```

```
// build.js
var js = require('atomify-js');

js({
  entry: __dirname + '/main.js',
  watch: true
}, function (err, src) {
  require('fs').writeFileSync('./bundle.js', src, 'utf8');
  console.log('bundle written');
})
```