Create a directive to download (cache) pluginfiles

We need a directive to handle plugin files instead of doing it in mm-format-text. This directive can be added manually (in img tags defined by ourselves) or automatically inside mm-format-text.

This comment shows an idea of how to do this.

Please notice that mm-format-text needs to be modified to add this directive when it's needed. To do so, something like this needs to be done:

angular.forEach(element.find('img'), function(el) {

    el.setAttribute('mm-no-input-validation', '');

    el = $compile(el)(scope);

});