Let a user copy the selected path

[yykamei]

It might be useful for a user to copy the currently selected path by typing some special keys. For example, I'm happy to copy a path by typing Ctrl + g on it.

  README.md
> My special file.txt -----> Copy this file name to clipboard!
  others.md

If It's implemented, I think feedback is required to show a user whether the copy command succeeds or not. I think it's should be shown on bottom status bar, which is not implemented so far.

[yykamei]

I want to skip the feedback for now.