Proposal: improve the plugins manager

[yosymfony]

Prior to Spress 1.1.0, to create a plugins without Composer is not easy: you need to create a .php file, a folder and a composer.json for your plugin and then, you need to register the namespace of your plugin in composer.json of your site. With this proposal, all .php files in _plugins folder will be loaded automatically with exception of files located at _plugin\vendors. Advantages:

• Write simple plugins without namespaces.
• Plugin located at plugins root or subfolder.
• Not need dump the composer autoloader for classloader.

"spress_class" key at "extra" section in composer.json:
This key become optional and only necessary for plugins with namespace.