Give the addfile command variable argument support.

This branch includes several improvements.

• 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"