- support page config define in front matter
- support `title`, which can set HTML title.
- if `title` is not set, then HTML title will try find page title defined in book.xml
- if `title` is set but <h1> title not set, it will generated a <h1> title using the value of `title`
- support `configurations` which can set page level configs here.
- now can open page that has .md file but not defined in book.xml (must in a book defined)
- old.title.gen is not supported yet.
