Allow the user to alter the bit grouping in hexl.el

Allow the user to alter the bit grouping in hexl.el

* hexl.el (hexl-bits): New variable.
(hexl-options): Mention the variable in the doc string.
(hexl-rulerise): New function.
(hexl-line-displen): New function
(hexl-mode): Mention the new variable.
(hexl-mode, hexl-current-address, hexl-current-address): Use the
displen.
(hexl-ascii-start-column): New function.
(hexl-address-to-marker, hexl-beginning-of-line, hexl-options)
(hexl-insert-char, hexl-mode-ruler): Use the displen.