Allow Sass files to be rendered, but never place them in layouts.

Fixes #2573.
Use the proper extname getter.
Make default rake task a multitask so tests run in parallel.
Merge pull request #2733 from jekyll/sass-in-liquid
Update history to reflect merge of #2733 [ci skip]
Release :gem: v2.3.0
Allow for parallel test execution.
Add Utils.has_yaml_header? so we can check the front matter existence in one place
Read in static files info `collection.files` as `StaticFile`s.
Merge pull request #2735 from jekyll/multitask-defaut
Update history to reflect merge of #2735 [ci skip]
Fix up script/cucumber and script/test
Custom formatter.
Add StaticFile#destination_rel_dir
Merge pull request #2737 from jekyll/fix-reading-imgs
Update history to reflect merge of #2737 [ci skip]
added sassify and scssify filters fot converting sass and scss strings to css
Merge pull request #2739 from DynamicDyno/sassify-filters
Update history to reflect merge of #2739 [ci skip]
Add some minimal docs for scssify and sassify filters. #2739 [ci skip]