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-src/SASS_CHANGELOG.md

@@ -62,7 +62,10 @@ Thanks to Alexander Pavlov for implementing this.
 * You can now detect the presence of a Sass feature using the new function
   `sass-supports($feature-name)`. There are no detectable features in this
   release, this is provided so that subsequent releases can begin to
-  use it.
+  use it. Additionally, plugins can now expose their functionality
+  through `sass-supports` by calling `Sass.add_feature(feature_name)`. Features
+  exposed by plugins must begin with a dash to distinguish them from
+  official features.
 
 ### Backwards Incompatibilities -- Must Read!
 

lib/sass/features.rb

@@ -19,6 +19,26 @@ module Features
     def has_feature?(feature_name)
       KNOWN_FEATURES.include?(feature_name)
     end
+
+    # Add a feature to Sass. Plugins can use this to easily expose their
+    # availability to end users. Plugins must prefix their feature
+    # names with a dash to distinguish them from official features.
+    #
+    # @example
+    #   Sass.add_feature("-import-globbing")
+    #   Sass.add_feature("-math-cos")
+    #
+    #
+    # @param feature_name [String] The case sensitive name of the feature to
+    #        check if it exists in this version of Sass. Must begin with a dash.
+    #
+    # @return [Boolean] whether the feature of that name exists.
+    def add_feature(feature_name)
+      unless feature_name[0] == ?-
+        raise ArgumentError.new("Plugin feature names must begin with a dash")
+      end
+      KNOWN_FEATURES << feature_name
+    end
   end
 
   extend Features

test/sass/functions_test.rb

@@ -978,10 +978,13 @@ def test_type_of
   end
 
   def test_sass_supports
-    Sass::Features::KNOWN_FEATURES << "my-test-feature"
-    assert_equal("true", evaluate("sass-supports(my-test-feature)"))
+    assert_raises ArgumentError do
+      Sass.add_feature("my-test-feature")
+    end
+    Sass.add_feature("-my-test-feature")
+    assert_equal("true", evaluate("sass-supports(-my-test-feature)"))
     assert_equal("false", evaluate("sass-supports(whatisthisidontevenknow)"))
-    assert_equal("true", evaluate("sass-supports($feature: my-test-feature)"))
+    assert_equal("true", evaluate("sass-supports($feature: -my-test-feature)"))
   end
 
   def test_unit