Revert "Fix a failing test"

This reverts commit 6f66b7b6bfcb68aac1acd937d4897aca6e554719.
Revert "Modify rendering Table of Contents in order to render hashes of invisible headers"

This reverts commit 6033f65450d72a294b077cdb0fe0549f005c326a.
Modify rendering table of contents in order to add CSS style for invisible headers
cleanup
Fix a failing test
Add Extension Mechanism
Update tests for the extension mechanism
Change code highlight library into "highlight.js"