This moves the `customeElements.define` calls from each respective
element and into the index.js file. This is useful for if we wish to
load the components classes without defining the custom element.

In addition, this changes the pattern of how we register the custom
elements, to allow for HMR, as we did in
github/catalyst#197.

…to-index-js-and-refactor

move customElements define calls to index.js and refactor

When requiring this module in NodeJS, it will error as `HTMLElement`
does not exist. Creating a variable defaulting to `null` allows the
class to be created in the server without causing errors. This is useful
for introspecting the class within NodeJS.

Ensure HTMLElement exists in all contexts

…ent-registration

add `define` export for custom element registration