preload fonts.css in base.tmpl

Hi, this is an "explorational" pull request.
PageSpeed Insights tells me I could gain about 500ms on my blog, if I preloaded fonts.css.
Would you be open to preloading fonts like this?
From a read of the templates and the generated HTML, it seems you always end up at the resource at {{.Host}}/css/fonts.css. The JS bit in the bottom just controls when to enable it. Preloading with a <link> element shouldn't change that behavior.

Let me know if that looks acceptable, I'm happy to throw additional commits in this pull request branch for the other template files.