Feat Req: support git blame

[liangkarl]

Hello @altsem,

Thanks for the great tools that helps to simplify git operations. However, I found there is a useful feature, git blame, that I couldn't find out in this tool.
I believe that many people would use git blame to trace code changes, so is it possible to support git blame in gitu?

Thanks

[altsem]

@liangkarl, i actually threw together a proof-of-concept in #527. You just press "B" on any file, and it can be repeated to navigate back in history. "q" to go back.