Fix #581 by implementing "skip state" that skips all meta-content sections the first time a book is played

This PR basically implements what I just wrote in the title.

The player object (`LYT.player`) now has a `inSkipState` property that reflects whether we're in skip state. This is decided in the `Lyt.player.load` function by checking if there's a `.lastmark` property on the book.

I need someone to review and test this code. I've tested on book 38737 which should like a charm, because that's the one I coded against. I'd also like to know if there are any other tags/classes in the NCC file that we'd like to regard as meta-content sections. This is however easy to extend.