dither link now creates per-file links in home directory.

eg if the build output directory contains .bashrc and .Xinitrc:
 - `dither link` first creates ~/.dither_dotfiles linking to the build output directory
 - `dither link` then creates ~/.bashrc -> ~/.dither_dotfiles/.bashrc and ~/.xinitrc -> ~/.dither_dotfiles/.xinitrc

Note if ~/.bashrc already exists and doesn't point to ~/.dither_dotfiles/.bashrc, `dither link` will move the existing ~/.bashrc to ~/.bashrc.moved_by_dither_at_<timestamp>.