Fix code to allow dots in variables

features/include_tag.feature

@@ -37,7 +37,8 @@ Feature: Include tags
   Scenario: Include a file from a variable
     Given I have an _includes directory
     And I have an "_includes/snippet.html" file that contains "a snippet"
-    And I have an "index.html" page that contains "{% assign file = 'snippet.html' %}{% include {{file}} %}"
+    And I have a configuration file with "include_file" set to "snippet.html"
+    And I have an "index.html" page that contains "{% include {{site.include_file}} %}"
     When I run jekyll
     Then the _site directory should exist
     And I should see "a snippet" in "_site/index.html"

lib/jekyll/tags/include.rb

@@ -76,7 +76,7 @@ eos
       end
 
       def retrieve_variable(context)
-        if /\{\{([\w\-]+)\}\}/ =~ @file
+        if /\{\{([\w\-\.]+)\}\}/ =~ @file
           raise ArgumentError.new("No variable #{$1} was found in include tag") if context[$1].nil?
           @file = context[$1]
         end