Child element toggle

When using this component in a project, I noticed that putting child elements within my h1-h6s would create "dead zones," that when clicked would not open accordion sections.  I am making a pull request to address that need as this seems like a very-common use case that is worth supporting.

The fix for this should have been pretty simple: change [this line](https://github.com/x-tag/accordion/blob/fc72bc51aa4b4e94688b21838a27283d4c9122ca/src/accordion.js#L29) from `select.call(event.target.parentNode, this);` to `select.call(this.parentNode, this);`.

However, I was surprised to see that it still wasn't working.  I found, after some experimentation, that using a class instead of a heading element seemed to work...which seems quite strange.  I'm not sure what's going on there, but that is the impetus for the "`.x-accordion-heading`" class.