Allow different class names

[nicholasio]

Currently, we need to use accordion-header, accordion-content, accordion-label classes.

It would be nice to allow the user the use other class names and pass those class names to the plugin via options maybe?

Caveat is they would need to use their own CSS.

[Firestorm980]

@nicholasio The PR linked adds settings for custom classes to the options. I wonder if we should change up the CSS so we can still have working accordions with custom classes by targeting them another way (data- maybe)?