Refactor and simplify extending by new add ons

I got rid of plugin configuration like 

    images: true,
    maps: true
    imagesOptions: {...}

and move to configuration like 

    plugins: {
      images: {...},
      maps: {...}
    }

Also, I generalize initialization process and now it doesn't depend on count of existing plugins and if you want to write new addon, you shouldn't replace code of main file (medium-editor-insert-plugin.js)