Add key-value typing for compatibility with reactstrap.

[huan086]

Some libraries takes [classname: string]: string and use classname if the mapping is not found. See reactstrap typing and reactstrap

[sarink]

I'm still hoping the original authors will begin paying attention to their repo again, although if they don't, maybe someday I'll declare this an official fork and merge some of the many open prs in here. Unfortunately at this time I'm not ready to commit to anything more than fixing this one issue