Add priority system to loader

[Aurelius7309]

This PR adds the option of modders to specify an integer priority value by appending a line to the Steamodded header as such:

--- PRIORITY: -100

Both the loading of mod files and execution of init functions have been adapted to adhere to an ordering of mods from lowest to highest priority values, i.e., a high negative priority value will cause your mod to be loaded first. Mods that don't specify a priority value are assigned a default value of 0, with the loading order of mods with equal priority staying unchanged.