Add rake task for testing theme during development

- `bundle exec rake preview`

Rakefile

@@ -0,0 +1,74 @@
+require "bundler/gem_tasks"
+require "jekyll"
+require "listen"
+
+def listen_ignore_paths(base, options)
+  [
+    /_config\.ya?ml/,
+    /_site/,
+    /\.jekyll-metadata/
+  ]
+end
+
+def listen_handler(base, options)
+  site = Jekyll::Site.new(options)
+  Jekyll::Command.process_site(site)
+  proc do |modified, added, removed|
+    t = Time.now
+    c = modified + added + removed
+    n = c.length
+    relative_paths = c.map{ |p| Pathname.new(p).relative_path_from(base).to_s }
+    print Jekyll.logger.message("Regenerating:", "#{relative_paths.join(", ")} changed... ")
+    begin
+      Jekyll::Command.process_site(site)
+      puts "regenerated in #{Time.now - t} seconds."
+    rescue => e
+      puts "error:"
+      Jekyll.logger.warn "Error:", e.message
+      Jekyll.logger.warn "Error:", "Run jekyll build --trace for more information."
+    end
+  end
+end
+
+task :preview do
+  base = Pathname.new('.').expand_path
+  options = {
+    "source"        => base.join('example').to_s,
+    "destination"   => base.join('example/_site').to_s,
+    "force_polling" => false,
+    "serving"       => true,
+    "theme"         => "minima"
+  }
+
+  options = Jekyll.configuration(options)
+
+  ENV["LISTEN_GEM_DEBUGGING"] = "1"
+  listener = Listen.to(
+    base.join("_includes"),
+    base.join("_layouts"),
+    base.join("_sass"),
+    options["source"],
+    :ignore => listen_ignore_paths(base, options),
+    :force_polling => options['force_polling'],
+    &(listen_handler(base, options))
+  )
+
+  begin
+    listener.start
+    Jekyll.logger.info "Auto-regeneration:", "enabled for '#{options["source"]}'"
+
+    unless options['serving']
+      trap("INT") do
+        listener.stop
+        puts "     Halting auto-regeneration."
+        exit 0
+      end
+
+      loop { sleep 1000 }
+    end
+  rescue ThreadError
+    # You pressed Ctrl-C, oh my!
+  end
+
+  Jekyll::Commands::Serve.process(options)
+end