Support for user modification of plugin config file

[wiiznokes]

I was thinking of a file named pop-launcher/plugins/{plugin}/plugin-user.ron or somethings like this. The service could parse this file after the default one.

Also, it is related to #220, but if we give an id for each query, the service could use it to parse the regex and send this id to the plugin. It will allows user to modify the regex field, without impacting plugins.