Merge pull request #216 from ianmcburnie/accessibility

Accessibility: added aria-labels to header
added documentation for theme option
Merge pull request #219 from BjoernSchotte/feature/doc-theme

added documentation for theme option
Fix #218: Add settings for client theme (font family, size, theme)
Don't show divider even if githubId is present.
Merge pull request #221 from frodsan/patch-1

Don't show divider even if githubId is present.
Force forward slashes on SUMMARY.md paths

this prevent windows user to generate invalid path on unix OS
Reference to issue #39 on gitbook-editor
Merge pull request #223 from cef62/fix-windows-paths

Force forward slashes on SUMMARY.md path for articles
Add infos about cover