Add automatic extraction of 7z files

[clin1234]

How much effort would be required to add 7z support, assuming Go's stdlib doesn't have it?

[dufferzafar]

This is what we should use: https://github.com/bodgit/sevenzip

eget's code is quite clean, this is where you can start:

https://github.com/zyedidia/eget/blob/0983deab36378a1536bf9dc97d5d56f6e5dd3477/extract.go#L71-L85

Not a lot of effort - you can do it @clin1234 ! 😃