New "FileWalker" for finding relevant files; added new Mistkerl LaTeX sniffers

FileWalker: os.walk()-alike method's where introduced. Since some times you want
to catch more files (e.g. LaTeX), or you want/want not to be restricted to
chapter prefixes, then this class can be used.

For many things in LaTeX, there are default shorthands. Report if some of them
are not used.