Client-v0.6.0

• Adding PEF validation support.
• Change to handle images within captions.
• Change to check everything depending on type instead of heading.
• Change to update the section depth instead of ignoring the heading level when we don't have an
  implicit one in the document.
• Handle the case when you have chapters that only have an aria label and no heading.
• EPUBCheck 5.0.0
• Adding HR into the allowed blocks so we can have HR in sidebars.
• Fixing issue with parts and chapters being in different files.
• Fixing locale issue with running validator.