[Fix #4] Add zsh-syntax-highlighting

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "plugins/history-substring-search/external"]
 	path = plugins/history-substring-search/external
 	url = https://github.com/zsh-users/zsh-history-substring-search.git
+[submodule "plugins/syntax-highlighting/external"]
+	path = plugins/syntax-highlighting/external
+	url = https://github.com/zsh-users/zsh-syntax-highlighting.git

plugins/syntax-highlighting/external

@@ -0,0 +1 @@
+Subproject commit bb0d575942760fe483790fe878ea43b1f781e2ca

plugins/syntax-highlighting/init.zsh

@@ -0,0 +1,12 @@
+#
+# Integrates zsh-syntax-highlighting into Oh My Zsh.
+#
+# Authors:
+#   Sorin Ionescu <sorin.ionescu@gmail.com>
+#
+
+if zstyle -t ':omz:plugin:syntax-highlighting' color; then
+  source "${0:h}/external/zsh-syntax-highlighting.zsh"
+  ZSH_HIGHLIGHT_HIGHLIGHTERS=(main brackets pattern cursor)
+fi
+