support optional format flag

lib/pixelpress/renderers/weasyprint_renderer.rb

@@ -1,7 +1,7 @@
 class Pixelpress::WeasyPrintRenderer
   def render(html)
 
-    command = "#{executable_path} --encoding UTF-8 - -"
+    command = "#{executable_path} --encoding UTF-8 #{format} - -"
 
     error = nil
     result = nil
@@ -13,16 +13,21 @@ def render(html)
       error = stderr.read
       process = thread.value
     end
-    raise "command failed (exitstatus=#{result.exitstatus}): #{command}\n #{error}" unless process.success?
+    raise "command failed (exitstatus=#{process.exitstatus}): #{command}\n #{error}" unless process.success?
     return result
   end
 
   def version
-    `#{executable_path} --version`.chomp.scan(/(\d+\.\d+)/).flatten.first
+    `#{executable_path} --version`.chomp.then { _1.scan(/(\d+\.\d+)/).flatten.first || _1.scan(/(\d+)/).flatten.first }
   end
 
   private
 
+  def format
+    puts "WeasyPrint version: #{version}"
+    "-f pdf" if version.to_f >= 51
+  end
+
   def executable_path
     `which weasyprint`.chomp || raise(StandardError, "could not find weasyprint")
   end