Convert rST block_quote to html5slides <q> elements

* rST supports quotes through the `block_quote` node (with an optional `attribution` child). The default HTML builders generate a `blockquote` element
* html5slides supports "quote" slides, but uses the `q` element (optionally followed with a `div[@class="author"]`)

Add a `block_quote` visitor to `BaseSlideTranslator` to generate a `q` element.

Restriction: only accept a single paragraph (required) and an optional attribution, html5slides's styling does not gracefully handles block elements for quotes, only inline elements in a `q`.

Fixes #72