Make Plugins use interpolated Grunt templates (e.g. BannerPlugin)

The BannerPlugin isn't interpolating the template I am passing in, which
will just about always be the case for someone using it.  I investigated
and it looks like the culprit is in grunt-webpack/tasks/webpack.js:

``` javascript
function getWithPlugins(ns) {
  var obj = grunt.config(ns);
  if(obj.plugins) obj.plugins =
    grunt.config.getRaw(ns.concat(["plugins"]));
  return obj;
}
```

This is clobbering the interpolated text with uninterpolated text.  I
understand why: `grunt.config()` turned the plugin objects into generic
Object instances and `grunt.config.getRaw()` restores them to plugin
instances, e.g. `BannerPlugin`.  The only problem is that grunt templates
end up being reverted too.

This fixes that issue by enumerating all the plugin properties and
processing any strings through `grunt.template.process()`.