Support custom file operations during ISO extraction

This could go something like this:

• User would create a text file with the same name as the ISO but with a specific extension. For instance, if you use en_windows_8_1_x64_dvd_2707217.iso you would create a en_windows_8_1_x64_dvd_2707217.ruf or something in the same directory
• The .ruf file would contain simple commands like:

d /sources/cversion.ini
r /sources/boot.wim C:/mydir/myboot.wim
i C:/mydir/EI.cfg /sources/EI.cfg

to delete, replace or insert a file onto the target USB during ISO extraction.