Add optional property `folderName` which rejects names containing any
invalid characters.
Rejected characters are `<` `>` `:` `"` `/` `\` `|` `?` `*` as required
by Windows and UNIX.
Does not protect against strings containing non-printable control
characters, or strings of Windows-reserved filenames like CON, PRN, AUX,
NUL. These checks should be done when creating the directory.
Also set the folderName parameter for mods that are known to be
sensitive to folder name: Cryptid, Talisman and Trance.
Brainstorm is another popular mod that requires a specific folder name,
but is not currently on the mod index.
