Add zplug as submodule

.gitmodules

@@ -0,0 +1,3 @@
+[submodule ".zplug"]
+	path = .zplug
+	url = https://github.com/zplug/zplug.git

.zplug

@@ -0,0 +1 @@
+Subproject commit 9fdb3889e0a131bf2af7bb5cc7ad91ccbf8213e4