Centralise management of external plugins repositories

[MarkHedleyJones]

Currently dmenu-extended has three urls hard-coded as indexes for plugins.
As plugins are spread over various repositories, it is hard to update those plugins to accommodate changes in this repository.
Or, in the unlikely event that some of the external urls abuse the ability to supply plugins to dmenu-extended users, they can't easily be disabled.

Options:

1. Request to merge third-party plugin repositories into the official dmenu-extended-plugins repository.
2. Create an index of plugin_urls that is fetched before scanning external plugin repositories.

[MarkHedleyJones]

Alternative option, define the plugin repositories as a list in dmenuExtended_preferences.txt.
This could also support local path references for plugin development.

By default only the official plugin repository would be used, but users can still add their own after installation.
Additionally, people can merge their plugins into the officialy repository to make them available to all users.