feat(divider): add implementation of the divider component

This pull request adds an implementation of the "divider" component which groups and separates content within lists and page layouts using strong visual and spatial distinctions. Demos, docs and a small test suite accompany the implementation.

Closes #194
What is the intent of the text inside of `material-divider`? It seems like it should be a heading, but the heading level depends on the surrounding content. For example, in this demo, `Folders` should be an h2 with the `material-item-content` headings changed to h3 and h4 so they create a content hierarchy.