Adding Textastic plugin

plugins/textastic/textastic.plugin.zsh

+#
+# If the tt command is called without an argument, launch Textastic
+# If tt is passed a directory, cd to it and open it in Textastic
+# If tt is passed a file, open it in Textastic
+#
+function tt() {
+    if [[ -z $1 ]]
+    then
+        open -a "textastic.app"
+    else
+        open -a "textastic.app" $1
+        if [[ -d $1 ]]
+        then
+            cd $1
+        fi
+    fi
+}