More configurable default options for pp

[scott-griffiths]

The width, sep and offset in pp all have defaults. These could be moved to

bitstring.options.pp_width = 120
bitstring.options.pp_sep = ' '
bitstring.options.pp_show_offset = True

This should be a separate feature request, but it's related: It would also be good to add a bit_width option to pp. This would specify the number of bits to show on each line, and would override the width. I'm finding that in practice this would be a more useful thing to have.