Fetch contributors image using GitHub API

[iamrajiv]

Instead of storing the contributor's images in the assets folder, it's better to fetch GitHub avatar images of contributors using GitHub API.

I have found these API:

https://avatars.githubusercontent.com/<username>
https://avatars.githubusercontent.com/u/<userid>

[huan]

Yes I agree that it's better to fetch GitHub avatar images for contributors!

I'd like to accept a PR to implement this great feature, thank you very much for the suggestion!