Add a links source directive to specify in which subfolder to search for files

[ealprr]

With a dotfiles repository organize with folder (previous GNU stow user for example) like:

dotfiles
   |-git
      |-.gitconfig
   |-vim
      |-.vimrc
      |-.vim/
   |-bash
   | ...

A directive links-src or links-from would be very usefull. Instead of:

[tasks.vim]
    links = [
        [".vimrc", "vim/.vimrc"],
        [".vim", "vim/.vim"],
    ]

One could write:

[tasks.vim]
    links-src = "vim"
    links = [
        [".vimrc"],
        [".vim"],
    ]