Add Language#find_by_extension

lib/linguist/language.rb

@@ -2,22 +2,42 @@ require 'yaml'
 
 module Linguist
   class Language
-    @name_index = {}
+    @name_index      = {}
+    @extension_index = {}
 
     def self.create(attributes = {})
       language = new(attributes)
 
       @name_index[language.name.downcase] = language
 
+      language.extensions.each do |extension|
+        @extension_index[extension] = language
+        @extension_index[extension.sub(/^./, '')] = language
+      end
+
       language
     end
 
+    def self.[](name)
+      find_by_name(name)
+    end
+
     def self.find_by_name(name)
       @name_index[name.downcase]
     end
 
-    def self.[](name)
-      find_by_name(name)
+    def self.find_by_extension(filename)
+      basename = File.basename(filename)
+
+      if basename[0] == ?.
+        ext = basename
+      elsif basename.include?('.')
+        ext = File.extname(basename)
+      else
+        ext = basename
+      end
+
+      @extension_index[ext]
     end
 
     attr_reader :name, :extensions