Add dev config for injecting HTML comments around function components

[chrismccord]

This would inject something like:

<!-- MyAppWeb.Components.flash lib/app_web/components.ex:125 -->
<div id="flash>
  ...
</div>
<!-- /MyAppWeb.Components.flash lib/app_web/components.ex:125 -->

Rationale: for a componentized app it can often become difficult to ascertain from the browser markup where it is being generated from. The module/func/file/line will be a nice dev UX boost.

Caveats: we can only inject comments when we see HTML tags inside the component.