Add doctype to list of self closing tags

I added upper and lowercase because that seemed
preferable to always casting every html function
name to lowercase.

src/compiler.js

@@ -12,6 +12,8 @@ module.exports = class Compiler {
       'br',
       'col',
       'command',
+      '!doctype',
+      '!DOCTYPE',
       'embed',
       'hr',
       'img',