feat: add options to customize manifest loader path

```js
// new options api
appCacheNanny.set(loaderPath, /path/to/my-custom-loader.html)
appCacheNanny.set({ loaderPath: /path/to/my-custom-loader.html })
appCacheNanny.get(loaderPath)
appCacheNanny.get() // returns all options
```

fixes #4