WORK IN PROGRESS: link step now links ~/.dither_dotfiles to installed_build

dither will first check if ~/.dither_dotfiles exists. If it does exist but doesn't point to installed_build, dither will politely move it aside to a filename like ~/.dither_dotfiles.moved_by_dither_at_<timestamp>.

Next steps: see previous commit for more detail, but generally, making links in ~ via ~/.dither_dotfiles for each of the built dotfiles. eg Linking ~/.bashrc -> ~/.dither_dotfiles/.bashrc, if our build contains .bashrc