[FEATURE] Allow setting file permissions for dotfiles

[svenstaro]

Is your feature request related to a problem? Please describe.
I'd like a way to specify final file permissions for my dotfiles. For instance, in git my permissions might be 644 but I might want to have them be 600 in the final location.

Describe the solution you'd like
A syntax such as suggested by you a few years back ssh = { source = "ssh", target = ".ssh", permissions = 700 }.

Describe alternatives you've considered
It would be possible to use hooks for this though I think that gets quite confusing as the permissions aren't clearly associated with the dot entries.

Additional context
#31 (comment)