add support for Browserify's opts.resolve

Browserify allows you to [override the default resolver](https://github.com/substack/node-browserify/blob/master/index.js#L76) (browser-resolve). 

Adding support for this feature.

e.g.:
```javascript
var myResolver = require('my-resolver');

module.exports = function(config) {
    config.set({
        // then later... 
        // Browserify config
        browserify: {
            files: ['./js/file/to/be/bundled.js'],
            resolve: myResolver
        },
        // ...etc...
    });
};
```