Batch convert multiple web pages into one e-book by URL, xml string, etc.
Features:
* Automatically generate cover: If the `<title>` text in html is one of [COVER_TITLE_LIST](https://github.com/dfface/xml2epub/blob/main/xml2epub/constants.py),
then the cover will be added automatically, otherwise the default cover will be generated. We will randomly generate the cover image with a similar "O'Reilly" style.
* Automatically obtain the core content of the article: we filter the obtained html string and retain the core content. See [SUPPORTED_TAGS](https://github.com/dfface/xml2epub/blob/main/xml2epub/constants.py) for a list of tags reserved in html.