Add some zsh plugins via git submodule

.gitmodules

@@ -0,0 +1,3 @@
+[submodule ".zsh.d/externals.d/zsh-syntax-highlighting"]
+	path = .zsh.d/externals.d/zsh-syntax-highlighting
+	url = git://github.com/zsh-users/zsh-syntax-highlighting.git

.zsh.d/externals.d/.placeholder

@@ -0,0 +1 @@
+# This file is a simple placeholder so that its parent directory will be tracked by git

.zsh.d/externals.d/zsh-syntax-highlighting

@@ -0,0 +1 @@
+Subproject commit d2715e2cb3294880bbc6a75b91288243054218d0

.zsh.d/zshrc

@@ -40,12 +40,19 @@ fi
 unset os
 
 # sourcing config files
-for f in $DOTZSHDIR/config.d/*.zsh; do
+for f in $DOTZSHDIR/config.d/*.zsh ; do
     if [ -r "$f" ]; then
         source $f
     fi
 done
 
+# sourcing externals plugins
+for d in $DOTZSHDIR/externals.d/*; do
+    if [ -f "$d/${d##*/}.zsh" ]; then
+        source "$d/${d##*/}.zsh"
+    fi
+done
+
 # using 'sudo zsh'
 if [ "$SUDO_USER" -a "$UID" = 0 ]; then
     # define root history path