Sass feature detection.

You can now call sass-supports($feature) in Sass files to check if Sass supports
some feature by name. This only returns true for the one major feature
added in the upcoming release so far: sourcemaps.
Don't have any known features in the 3.3 release.
Better documentation for the has_feature? method.
Allow plugins to expose their features too.

Plugins can call Sass.add_feature(feature_name) to expose their features
and in doing so get the same feature detection benefits that Sass itself
gets from this capability. Plugin features must begin with a dash to
distinguish them from official features and ensure our namespace is not
polluted.
Doc cleanup for the features module.
Rename sass-supports() to feature-exists().
Document feature-exists() in the Sass functions reference.
Style: multiline YARD directives are 2-space indented.
Cleanup test state for feature detection.
Merge branch 'master' into feature_detection

Conflicts:
	doc-src/SASS_CHANGELOG.md
Merge pull request #796 from nex3/feature_detection

Feature detection