mask.c: Allow escaping \ as \\. So \\x40 means literally \x40 and not
the character with ascii code 0x40.
Allow custom masks given in UTF-8 on command line.
mask.c: Allow using static placeholders within custom placeholders
?1..?4.

# three-letter-words, capitalized or not
$ ../run/john -4=?u?l -mask=?4?l?l -stdout

# hex digits 00-ff
$ ../run/john -4=[?da-f] -mask=?4?4 -stdout