Gives the addfile command the ability to add multiple files at the same time, using multiple arguments
Adds a check to see if the file was already added during the adding of multiple files, so that addfile a a a a a a a a a (assuming the file a doesn't exist) doesn't lag the crap out of your SRB2
Adds a warning if the file is skipped due to being already added during the adding of multiple files
Updates the help message to mention Luas and PK3s, and changes the description to "load mods"
