adds filter option for filename tweaks

While working with browserify, I've found it useful to prefix files that I don't want being compiled with an underscore like `_avatar.js` or `_description.js` mentioned in your readme. However, I don't want to have to have the underscores in the require statements like `require('_avatar.js');`. I couldn't find a way to do this as of now, so I added an option with the task to tweak file names and add them to `expandedAliases`.

For example:


```js
b.plugin(remapify, [
  {
    src: './**/*.js'
    , filter: function(fileName) {     // fileName is passed as something like `./foo-dir/_bar-file.js`
      return (/\/?((?:.(?!\/))+)$/)    // regex for everything after the last '/' in the file name
                .exec(fileName)[1]     // get file name from `exec` method
                .replace(/^\_/, '')    // replace leading underscore
                .replace(/\.js$/, ''); // replace file extension
    }
  }
]);
```

I played around with using the `remapify:file` event, but I felt like this was a cleaner way. What are your thoughts on this? Is there an easier way that I missed?