New features for arbitrary sites
New features for arbitrary sites

* next_selector: find next content page, if not using chapter selector
* content_title_selector: pull a chapter title from the content
* content_text_selector: pull specific text from the content element

`content_selector` will now fetch all content elements on the page, each
as a Chapter, not just the first one that matches.

Zomega/leech,kemayo/leech