Support loading configuration from local file for webpack instance

[sohkai]

Rather than trying to support everything out of the box for users (a la #21), it'd be super nice if the webpack instance could load configuration options, like loaders and plugins, from a user specified file. I assume lots of people roll their own way to deal with js/css these days and have already done the configuration that's needed in their own webpack configs.

I haven't looked too closely at how to do this, but I assume it won't be too hard to tell the current webpack config to grab a different one and merge it. I'll try to fiddle around with it and send a PR.