Ability to use rem for top and bottom offset

[prescottbreeden]

Really love this library but is it able to take rem for top and bottom offsets? My initial attempts don't seem to be successful and I couldn't see an option in the documentation besides % and px which unfortunately are not ideal for my situation.

If it can't, perhaps a feature for consideration, thanks.

[Nantris]

@lencioni @trotzig any thoughts? Would you be open to a PR to add this? We also need rem support. We can sort of shoehorn rem in within the context of our project, but it would be great if React Waypoint supported them natively.