Add startPage option support for arbitrary initial page

Historically bookblock rendered the first page first. This adds support for the client to specify what page to initially render.

This is useful for situations where it is desired to link directly to a slide by using the hash of the URL.

### NOTE:

It was not clear to me how the minified files get built. There does not appear to be any build file in this project to generate those. So if this gets pulled in, that will need to happen.