Add Jekyll::AutolinkEmail and Jekyll::GitMetadata

Add these plugins to the list of plugins.
Merge pull request #2596 from ivantsepp/add_ivantsepp_plugins
Update history to reflect merge of #2596 [ci skip]
Fix Textile link

Fix broken link to Textile site to point to RedCloth
Fix loads of other broken links
Add ids to history blocks which are useful.
Add slugs to the release sub headers, too!
Merge pull request #2601 from alfredxing/master
Update history to reflect merge of #2601 [ci skip]
Add script/proof to cibuild
Proof site with html-proofer

Proof site with html-proofer when the site directory is changed
Uncomment the exit
Simpler way to install nokogiri with system extensions
Merge pull request #2605 from alfredxing/proof-site
Update history to reflect merge of #2605 [ci skip]
Cleaning up the cibuild stuff.
Run bundle exec rake tho.
Fix up `grep` pattern

Change from `^/site` to `^site/`
Fix `grep` pattern in `script/test` as well
Merge pull request #2608 from alfredxing/fix-proof-grep