Add ability to set row length in hexl-mode

[Umberto Tozzato]

Hello,
currently there is way to change the legth of the rows in hexl-mode (defaults 
to 1 byte / 16 ASCII chars per line).

I think there should be a way to set an arbitrary row lenght as many binary 
files have special alignment which only makes sense if you allign each line at 
a certain number of characters.

--
Umberto