This implements RSS timeline generation in gmnhg.
RSS is generated both for the whole site, and for the content
directories as an rss.xml file inside these directories.
RSS requires the absolute URI to the article. For this to work,
a geminiBaseURL setting is required to be set in the Hugo
configuration file (config.toml/json/yaml).
RSS template can be ovewritten on the site-wide /
directory-wise basis; see godoc on how to do this.
As there's no discovery method of an RSS timeline in Gemini,
the users are expected to put a link to rss.xml on their site
where necessary.
