support split chapter by 'div'
enhance chapter split and other minor improvement

Chapter split support "<div class='makeepub-chapter-level*X*>Title</div>"
where X is a number between 1 and 6. The level of the chapter will be X,
and the title of the chapter will be 'Title'.

Enhance configuration reading to support mulit-line options.

New supported EPUB properties: language, publisher, description
enhance chapter split and minor refactor
refactor for new functionality in progress