Beefs up variables.md

Adds site.data
Adds site.collections
Adds site.html_pages
Release post for 2.1.0
Bump version in gem.
update history to reflect release of 2.1.0
Booooom, 2.1.0 in history on site.
Fill in a bit of missing TomDoc

Fill in a piece of missing doc for the `build` function in `commands/build.rb`
Merge pull request #2553 from jekyll/release-2.1.0

Release 2.1.0
Merge pull request #2554 from alfredxing/fix-build-tomdoc
Update history to reflect merge of #2554 [ci skip]
add #2554 to history on site
Add 3 more lines to 2.1.0 release post.
Release 2.1.0
Add contributor list to 2.1.0 release post.

I forgot! I'm really sorry!
Minor improvements to the rakefile.
Add vertical margin to `highlight`

…so multiple code blocks are distinctive.
Merge pull request #2558 from kleinfreund/patch-1
Update history to reflect merge of #2558 [ci skip]
Fix typo
Merge pull request #2561 from jshawl/fix-typo

Fix typo
Add feature for layouts vuln.