Allow parser and compiler classes to be set in options

This makes it easier to extend and modify Haml. The change was
originally motivated by issue #570.

CHANGELOG.md

@@ -13,7 +13,8 @@
 
 * Haml's internals have been refactored to move the parser, compiler and options
   handling into independent classes, rather than including them all in the
-  Engine module.
+  Engine module. You can also specify your own custom Haml parser or compiler
+  class in Haml::Options in order to extend or modify Haml reasonably easily.
 
 * The :sass filter now wraps its output in a script tag, as do the new :less and
   :scss filters. The :coffee filter wraps its output in a script tag.

lib/haml/engine.rb

@@ -1,10 +1,10 @@
 require 'forwardable'
 
+require 'haml/parser'
+require 'haml/compiler'
 require 'haml/options'
 require 'haml/helpers'
 require 'haml/buffer'
-require 'haml/parser'
-require 'haml/compiler'
 require 'haml/filters'
 require 'haml/error'
 
@@ -60,8 +60,8 @@ module Haml
 
       initialize_encoding options[:encoding]
 
-      @parser   = Parser.new(@template, @options)
-      @compiler = Compiler.new(@options)
+      @parser   = @options.parser_class.new(@template, @options)
+      @compiler = @options.compiler_class.new(@options)
 
       @compiler.compile(@parser.parse)
     end

lib/haml/options.rb

@@ -19,7 +19,9 @@ module Haml
       :remove_whitespace    => false,
       :suppress_eval        => false,
       :ugly                 => false,
-      :cdata                => false
+      :cdata                => false,
+      :parser_class         => ::Haml::Parser,
+      :compiler_class       => ::Haml::Compiler
     }
 
     @valid_formats = [:html4, :html5, :xhtml]
@@ -160,6 +162,12 @@ module Haml
     # xhtml.
     attr_accessor :cdata
 
+    # The parser class to use. Defaults to Haml::Parser.
+    attr_accessor :parser_class
+
+    # The compiler class to use. Defaults to Haml::Compiler.
+    attr_accessor :compiler_class
+
     def initialize(values = {}, &block)
       defaults.each {|k, v| instance_variable_set :"@#{k}", v}
       values.reject {|k, v| !defaults.has_key?(k) || v.nil?}.each {|k, v| send("#{k}=", v)}