add option for edge fading

[QuentinSc]

Hello,

I just discovered the package https://pub.dev/packages/fading_edge_scrollview and I love it. I propose an option to use it in your fabulous widget. I tried to implement it without complexify maintenance. I added it to example.

Thank you

[QuentinSc]

Don't hesitate to criticise my work, it's my first pull request on flutter :)

[iamvivekkaushik]

Awesome work @QuentinSc, can you try to implement edge fading without using another third-party library? I am a little busy these days so I couldn't review your pull request sooner.

[QuentinSc]

Thank you. I correctly reposted my gitlab comment. And I must correct my code, in english it's "fading" and not "fadding" ^^. I could implement it without third party but I don't know when