Set Pug's doctype option to html by default

[aantipov]

Pug's doctype option is set to html by default in pug-plain-loader.
The reasoning is also explained there:

The doctype option is set to html by default, since most Vue templates are HTML fragments without explicit doctype.

The same we should do here in component-compiler-utils (otherwise developers should figure it out and set it by themesevles vuejs/vue-jest#170)

I can create a PR if we agree to do it