Adding support for swapping out the parse function.

This can be used to cache parsed liquid templates, closing chamnap#18

README.md

@@ -113,6 +113,18 @@ Check out more [examples](https://github.com/yoolk/liquid-rails/blob/master/spec
 
 It works for any ORMs. The PORO should include `Liquid::Rails::Droppable`. That's all you need to do to have your POROs supported.
 
+### Caching Compiled Templates
+
+In order to cache compiled templates, you may do something like the following in your application.rb (using the excellent `concurrent-ruby` gem):
+```
+cached_templates = Concurrent::Map.new do |m, template|
+  Rails.logger.info "Compiling Template"
+  m[template] = ::Liquid::Template.parse(template)
+end
+app.config.liquid_rails.render_method = :render!
+app.config.liquid_rails.parse_template = lambda { |template| Rails.logger.info("compiling new template"); cached_templates[template] }
+```
+
 ### RSpec
 
 In spec_helper.rb, you'll need to require the matchers:

lib/liquid-rails.rb

@@ -11,7 +11,7 @@ module Rails
     autoload :Drop,             'liquid-rails/drops/drop'
     autoload :CollectionDrop,   'liquid-rails/drops/collection_drop'
 
-    Configuration = Struct.new(:render_method).new
+    Configuration = Struct.new(:render_method, :parse_template).new
 
     def self.setup_drop(base)
       base.class_eval do

lib/liquid-rails/template_handler.rb

@@ -17,6 +17,11 @@ def render_method
         (::Rails.env.development? || ::Rails.env.test?) ? :render! : :render
       end
 
+      def parse(template)
+        return Configuration.parse_template.call(template) if Configuration.parse_template
+        Liquid::Template.parse(template)
+      end
+
       def render(template, local_assigns={})
         @view.controller.headers['Content-Type'] ||= 'text/html; charset=utf-8'
 
@@ -28,7 +33,7 @@ def render(template, local_assigns={})
         assigns['content_for_layout'] = @view.content_for(:layout) if @view.content_for?(:layout)
         assigns.merge!(local_assigns.stringify_keys)
 
-        liquid = Liquid::Template.parse(template)
+        liquid = parse(template)
         liquid.send(render_method, assigns, filters: filters, registers: { view: @view, controller: @controller, helper: @helper }).html_safe
       end