Added index.html pattern

[wrabit]

This PR introduces ability to choose a default component when calling a directory path by using an index.html.

Before:

- cotton
  - comp
    - header.html
    - main.html


<c-comp.main>
  <c-comp.header />
</c-comp.main>

After:

- cotton
  - comp
    - header.html
    - index.html


<c-comp>
  <c-comp.header />
</c-comp>