Allow campaigns to include configs

Campaigns have always had the limitation that they can't execute any configs. Most notably, this doesn't allow you to set your own physics and balance for your campaign. My branch fixes that issue, by allowing both a global campaign cfg as well as a per level campaign cfg.

Example: If you have the campaign file maps/campaign_xonotic.txt the file maps/campaign_xonotic.cfg will be executed when any level of the campaign is ran. You can also create the file maps/campaign_xonotic_3.cfg which will only be executed when you're playing level 3 of that campaign.