Provide option to set the default doctype name

Author: kylekatarnls

src/Phug/Compiler.php

@@ -113,6 +113,7 @@ public function __construct(array $options = null)
             'basedir'              => null,
             'extensions'           => ['', '.pug', '.jade'],
             'default_tag'          => 'div',
+            'default_doctype'      => 'html',
             'pre_compile'          => [],
             'pre_compile_node'     => [],
             'post_compile'         => [],

src/Phug/Compiler/DoctypeCompiler.php

@@ -19,6 +19,8 @@ public function compileNode(NodeInterface $node, ElementInterface $parent = null
             );
         }
 
-        return new DoctypeElement($node->getName());
+        $name = $node->getName() ?: $this->getCompiler()->getOption('default_doctype');
+
+        return new DoctypeElement($name);
     }
 }