feat(marktext): add plugin

plugins/marktext/README.md

+## marktext
+
+Plugin for MarkText, a previewer for Markdown files on Mac OS X 
+
+### Requirements
+
+ * [MarkText](https://github.com/marktext/marktext)
+
+### Usage
+
+ * If `marktext` is called without an argument, open MarkText
+
+ * If `marktext` is passed a file, open it in MarkText
+
+### Credits
+
+ * just copied from plugins/marked2, all credits to marked2 plugin author

plugins/marktext/marktext.plugin.zsh

+#
+# If marktext is called without an argument, open MarkText
+# If marktext is passed a file, open it in MarkText
+#
+function marktext() {
+    open -a "MarkText.app" "$1"
+}