Support passing Liquid variables to includes

Change the regex matching to allow Liquid variables and object fields
to be passed to the include. Use the render context to retrieve the
variable value. Also, relax syntax checks by allowing surrounding spaces
and dashes in parameter names.

lib/jekyll/tags/include.rb

@@ -2,7 +2,7 @@ module Jekyll
   module Tags
     class IncludeTag < Liquid::Tag
 
-      MATCHER = /(\w+)=(?:"([^"\\]*(?:\\.[^"\\]*)*)"|'([^'\\]*(?:\\.[^'\\]*)*)')/
+      MATCHER = /([\w-]+)\s*=\s*(?:"([^"\\]*(?:\\.[^"\\]*)*)"|'([^'\\]*(?:\\.[^'\\]*)*)'|([\w\.-]+))/
 
       def initialize(tag_name, markup, tokens)
         super
@@ -16,7 +16,7 @@ module Jekyll
         end
       end
 
-      def parse_params(markup)
+      def parse_params(markup, context)
         params = {}
         pos = 0
 
@@ -42,6 +42,8 @@ eos
             value = match[2].gsub(/\\"/, '"')
           elsif match[3]
             value = match[3].gsub(/\\'/, "'")
+          elsif match[4]
+            value = context[match[4]]
           end
 
           params[match[1]] = value
@@ -67,7 +69,7 @@ eos
             partial = Liquid::Template.parse(source)
 
             context.stack do
-              context['include'] = parse_params(@params) if @params
+              context['include'] = parse_params(@params, context) if @params
               partial.render(context)
             end
           else