Enhancement: Plugin Manager

ref. to https://github.com/Codiad/Codiad/issues/376

Currently, I am unable to find any issue, so I wanted to add this as a pull request for deeper testing.

- Plugin Market
- For automatic installation/updates PLUGINS directory has to have write permissions
- added ```exclude``` functionallity for the update operation

plugin,json could now have something like ```"exclude": "config.php,otherconfig.php,foldername"```, which will not be updated automatically

Note: This request has no need to be merged quickly, this is more opened for testing purposes.