Add margins only if $location is provided

Before this commit, when using isolation, you were forced to pass a $location to span(), yet if you passed 0 you got no margin-left (in ltr) in return, this commit:
* Allows you to omit the $location argument if you don't want to mess with source order (leaving you with the standard float method, if I understand correctly).
* Allows you to pass 0px/% (can't be unitless, possibly a bug) and get the appropriate margin in return.

This works fine in my tests, but please run a few more to make sure I don't introduce any regression.