Stackedit modularization: Plugins folders

To follow up on #542, this branch provides a modularized structure for loading plugins into Stackedit

Usage: PLugins are now defined in the `/public/plugins` folder and are activated one place only: in the file ` public/res/plugins.js` (and only once in this file as opposed to previously)

Work done:
* [x] All providers moved into new plugins structure
* [x] All specific extensions moved into one place: `public/res/plugins.js`
* [x] All specific extensions removed from `publisher.js`, `synchronizer.js`, `sharing.js`

But it still works! 

How it works: All plugins are registered in `public/res/plugins.js`, which now loads each of the defined plugins by sending them to Publisher, Synchronizer, Sharing for initialization. After that the `onPluginsLoaded` event is submitted.


This fully solves the means of #542 but could be improved further so that for instance html snippets relevant for each plugin can be put in that specific directory and loaded only if relevant.