be able to add a custom class to a quote

[nticaric]

Currently, it's not possible to add custom classes to a single quote which might be useful for styling. In my case, I want the quotes to be compatible with bootstrap so I need to be able to add a col-md-x class to a single quote.

This pull request allows this.

[jeffikus]

@mendezcode i think this is ok with the exception of adding more sanitization on the class output. You could literally inject anything in there right now. I would say add something like esc_attr() around it.