Issue description

Currently there is no way to specify offset 0. This means that if you choose to use an offset at a smaller size and then remove it at another breakpoint you can't. Modifying bootstrap classes not through React strap will get the desired results with offset-0

Steps to reproduce issue

• Create a Col and give it an offset of one of greater at xs,sm or md
• Try and reset using offset-lg-0

Reseting of offset is ignored.