Feature changes

• mandown get ... --split-by-chapters now includes the chapter number in the title
• Chapter slugs now are automatically ordered in the format 00001. Chapter 1 if it doesn't sort itself nicely
• CBZ metadata now follows the ComicInfo.xml standard to split chapters

Bug fixes

• Sources no longer accessible are no longer included in Mandown.
• Chapter and title slugs are now checked and stripped of leading and trailing spaces before being written to disk (#102)