Add predefined file structures

An idea: having "profiles" that will automatically generate prefix/suffix bytes to make your resource look like a GIF, PNG, PDF, whatever. Something like `$ wrapwrap --profile gif:100:100 /etc/passwd` would generate a PHP filter that makes /etc/passwd look like a 100 by 100 GIF. The tool is not used enough to justify spending time on this yet, but maybe in the future.