Accessible accordion

updated the template to valid html semantics

added accordionTranscludeId directive so that by setting the id on accordion-group, it sets the heading link to the content's id

@bc-edlou @SiTaggart @alexmckenley 
I'm pretty sure that adding accordionId here makes it a required attribute for every instance of this directive. You could make it optional by adding `?@`  instead of `@` (I think). But you would have to also make sure to handle the case where it is undefined wherever it is used..