Support 'doctype 5'

**Slim** supports not only `doctype html` but also `doctype 5` for `<!DOCTYPE html>`. Let's **Slm** does so too!

###### References:

1. http://rdoc.info/gems/slim/file/README.md#Doctype_tag
1. https://github.com/judofyr/temple/blob/master/lib/temple/html/fast.rb#L7