Support native __webpack_public_path__.

This means you don't need to set the `publicPath` option during setup and the loader will use webpack's native one. Resolves faceyspacey#51.

Author: izaakschroeder

loader.js

@@ -54,10 +54,10 @@ module.exports.pitch = function(request) {
 		}
 
 		var childFilename = "extract-text-webpack-plugin-output-filename"; // eslint-disable-line no-path-concat
-		var publicPath = typeof query.publicPath === "string" ? query.publicPath : this._compilation.outputOptions.publicPath;
+		var publicPath = typeof query.publicPath === "string" ? query.publicPath : null;
 		var outputOptions = {
 			filename: childFilename,
-			publicPath: publicPath
+			publicPath: publicPath || this._compilation.outputOptions.publicPath,
 		};
 		var childCompiler = this._compilation.createChildCompiler("extract-text-webpack-plugin", outputOptions);
 		childCompiler.apply(new NodeTemplatePlugin(outputOptions));
@@ -140,12 +140,14 @@ module.exports.pitch = function(request) {
 					// All we need is a date that changes during dev, to trigger a reload since
 					// hashes generated based on the file contents are what trigger HMR.
 					if (process.env.NODE_ENV === 'development') {
+						const pathVar = publicPath ?
+							`"${publicPath}"` : "__wepback_public_path__";
 						resultSource += `
 if (module.hot) {
 	module.hot.accept();
 	if (module.hot.data) {
 		var neverUsed = ${+new Date()}
-		require(${loaderUtils.stringifyRequest(this, path.join(__dirname, "hotModuleReplacement.js"))})("${publicPath}", "%%extracted-file%%");
+		require(${loaderUtils.stringifyRequest(this, path.join(__dirname, "hotModuleReplacement.js"))})(${pathVar}, "%%extracted-file%%");
 	}
 }`;
 					}