Added the very useful .animation-fill-mode() mixin

I think that's the only animation property that was missing and it's quite useful.

https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode
Add docs.css in the CSSComb task.
Run csscomb before cssmin.
Merge pull request #12861 from twbs/docs-csscomb

Add docs.css in the CSSComb task.
Break csslint targets.
Fix csslint errors.
Move csslint overrides in Gruntfile.js.

This works around https://github.com/gruntjs/grunt-contrib-csslint/issues/26
Add sitemap.xml and robots.txt files.
Fixes #12851: it's not a tumah
Merge pull request #12866 from twbs/sitemap

Add sitemap.xml and robots.txt files.
Merge pull request #12855 from lipis/patch-1

Added the very useful .animation-fill-mode() mixin
100% less tumah
Gruntfile.js: Remove extra comma.
Fix small typo in JavaScript docs
Merge pull request #12880 from hnrch02/patch-1

Fix small typo in JavaScript docs
Update less.js to v1.7.0.
Update grunt-contib-less.
Run `grunt`.
Remove "bootstrap" keyword
Merge pull request #12889 from ZDroid/composer-json-keywor

Remove "bootstrap" keyword from composer.json