request: minimize overlap or avoid full overlap in cbc_design()

[shigono]

Hello,

In designs generated by cbc_design(methods = random|full|orthogonal|dopt), overlapped levels in a same question is not minimized. It could be a problematic, especially in random design.
It could be very helpfull if we can minimize overlap, or avoiding full overlap of any one attribute.
(this request is based on #27 (comment))

Thank you so much,
Shigeru