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.
Fix an @extend trimming bug.

Closes #788.
Merge branch 'stable'
Change the source map comment character from "@" to "#".

Closes #795
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.
Define Bool::TRUE and Bool::FALSE constants

Instead of generating a different sass object for each boolean defined
and returned during the processing of Sass files, we now define TRUE and
FALSE constants and return those instead.
Merge remote-tracking branch 'origin/stable' into stable
Merge branch 'stable'
Merge remote-tracking branch 'origin/master'
Allow negative indices in `nth` list function

Allow negative indices between -1 and (-1 * list length) in the `nth`
function.
Add explanation of using negative indices
Merge pull request #808 from ggilder/nth-negative-indices

Allow negative indices in `nth` list function
Documentation and code-style cleanup.
Merge pull request #797 from nex3/bool_constants

Use Bool::TRUE and Bool::FALSE constants to reduce GC needs.
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.