add backup script, cleanup link script

* add initial stab at a backup script to copy existing files to a backup directory
* clean up unused code in link.sh

install/backup.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+# Backup files that are provided by the dotfiles into a ~/dotfiles-backup directory
+
+DOTFILES=$HOME/.dotfiles
+BACKUP_DIR=$HOME/dotfiles-backup
+
+set -e # TODO: what does this do?
+
+echo "Creating backup directory at $BACKUP_DIR"
+mkdir -p $BACKUP_DIR
+
+linkables=$( find -H "$DOTFILES" -maxdepth 3 -name '*.symlink' )
+
+for file in $linkables; do
+    filename=".$( basename $file '.symlink' )"
+    target="$HOME/$filename"
+    if [ -e $target ]; then
+        echo "backing up $filename"
+        cp $target $BACKUP_DIR
+    fi
+done
+
+typeset -a files=($HOME/.config/nvim $HOME/.vim $HOME/.vimrc)
+for file in $files; do
+    if [ -e $file ]; then
+        cp -rf $file $BACKUP_DIR
+    fi
+done