In WebpackDevMiddleware, support array-style Webpack configs #291

As per comments in #132, Webpack itself allows multiple simultaneous configurations (the webpack.config.js file just needs to export an array of configs).

WebpackDevMiddleware should support it (just because there's no reason why not, as long as each config specifies a distinct publicPath). Need to have the JS code return an array of publicPath values to .NET rather than just a single one.