Add Pathname#mime_type

lib/linguist/pathname.rb

@@ -1,4 +1,5 @@
 require 'linguist/language'
+require 'mime/types'
 
 module Linguist
   class Pathname
@@ -28,6 +29,13 @@ module Linguist
       language.lexer
     end
 
+    def mime_type
+      @mime_type ||= begin
+        guesses = MIME::Types.type_for(extname)
+        guesses.first ? guesses.first.simplified : 'text/plain'
+      end
+    end
+
     def to_s
       @path.dup
     end