Wrong class resolving

[senyaak]

#https://github.com/caseyWebb/selectr/blob/12d92a56590791f7aa021fc56e7ae02b610da315/src/selectr.coffee#L47 If the multiple attribut is not set on the element - the "undefined" class will be added. It is in all cases where string interpolation is used. I would make a pr, but I didn't have worked with coffee script, so it would be nice if somebody fix that)

[senyaak]

PS I would be nice too if you could add classes to element