Use a relative path for the declaration assets (#214)

This will resolve the declaration issue.

index.ts

@@ -496,7 +496,8 @@ function ensureTypeScriptInstance(loaderOptions: LoaderOptions, loader: any): {
                 let output = languageService.getEmitOutput(filePath);
                 let declarationFile = output.outputFiles.filter(filePath => !!filePath.name.match(/\.d.ts$/)).pop();
                 if (declarationFile) {
-                    compilation.assets[declarationFile.name] = {
+                    let assetPath = path.relative(process.cwd(), declarationFile.name);
+                    compilation.assets[assetPath] = {
                         source: () => declarationFile.text,
                         size: () => declarationFile.text.length
                     };