Unsupported characters break XML files

[Ana06]

I used addlicense to add license + copyright header to FLARE-VM in mandiant/flare-vm#638. The config.xml file had a <feff> (Byte Order Mark (BOM)) that caused that addlicense added the header in the wrong way, breaking the file (and FLARE-VM). Support this type of characters to avoid issues in the future. There is a PR already with a proposed fixed in #166.