Add install.zsh for standalone installs

zshrc

@@ -3,8 +3,12 @@
 
 # Load plugin scripts
 source-plugin() {
-  if [ -d ~/.config/zsh/$1 ]; then
-    source ~/.config/zsh/$1/$1.plugin.zsh
+  local shared_plugin=~/.local/share/zsh/plugins/$1/$1.plugin.zsh
+  local local_plugin=~/.config/zsh/$1/$1.plugin.zsh
+  if [ -f $shared_plugin ]; then
+    source $shared_plugin
+  elif [ -f $local_plugin ]; then
+    source $local_plugin
   else
     echo "zsh plugin not found: $1"
   fi