Add placeholder plugin

I've created a placeholder plugin for TinyMCE. I believe this is a useful feature to have, whenever templates (eg email templates) should be editable for people who aren't too familiar with templating languages/engines. Its usage is pretty self-explanatory, but in case some thing is not quite clear, I first created a separate repo for this plugin here: https://github.com/EVODelavega/placeholder-plugin. 

Usage of the plugin is explained in the readme and the examples in that repo.