more flexible class selection

Currently radiobuttons_convert_element only works if the radio element being replaced has only the class "content" assigned to it, because that value is hard-coded into the javascript file. By adding a new option, we can enable other radio elements (such as pricesets, which have "contribution_amount-content" specified as an additional class). Without this change, the extension can still replace the radio element with buttons but the feature that indicates that an option is selected (and changes the background color) never matches.