Add html import stub generation support

A config might look like this:

stubs_to_generate:
  core_elements:
    - core-collapse/core-collapse.html
    - core-drawer-panel/core-drawer-panel.html
    - core-header-panel/core-header-panel.html

String is there to support

stubs_to_generate:
  core_elements: core-collapse/core-collapse.html

but that might be overkill