Features

• add default font
• add support of changing column gap
• add support for setting min column width
• add support for letter spacing
• add support for setting font weight
• seperate margin header and footer
• seperate margin side into margin start and margin end

Fixes

• fix background images are not shown for some ebooks
• fix current location not preserved when jumping around by link
• fix incorrect color of window buttons(min, max, close) on Windows