Add support for Google Analytics

Add Disqus-integration

Fixes EmielH#38

Remove unused i18n strings

Since EmielH#29, these strings are no longer used.

Update Hugo test versions

Update Hugo test versions

Add tests for Hugo 0.55.1

Add option to include additional CSS files

Put additional CSS files in the static folder of your site and add the
filenames to config.toml in the css parameter.

See EmielH#13

Add tests for Hugo 0.54.0