Ability to set the proxy URL and the metadata object

[dudidayan1]

Hi,
I love this library!

I want to ask for some improvements:

1. add the ability to set the proxy URL
2. I want to cache the preview results (the metadata JSON), so it can be nice to pass the fetched object itself instead of the fetcher.
3. Add onFailure callback which being called when failed to get the metadata

Thanks

[Dhaiwat10]

Hey, please feel free to put in a PR for these features if you want and I will merge it :)