feat: Expose document categories as a single rss category

An RSS category can be hierarchical by separating the individual parts
with a forward slash.

https://www.rssboard.org/rss-specification#ltcategorygtSubelementOfLtitemgt

Author: vbfox

src/document.rs

@@ -79,19 +79,24 @@ impl Document {
             .permalink(true)
             .build();
 
+        let mut categories = self
+            .front
+            .tags
+            .iter()
+            .map(|c| Category::from(c.as_str()))
+            .collect::<Vec<_>>();
+
+        if !self.front.categories.is_empty() {
+            categories.push(self.front.categories.join("/").into());
+        }
+
         let item = rss::ItemBuilder::default()
             .title(Some(self.front.title.as_str().to_owned()))
             .link(Some(link))
             .guid(Some(guid))
             .pub_date(self.front.published_date.map(|date| date.to_rfc2822()))
             .description(self.description_to_str())
-            .categories(
-                self.front
-                    .tags
-                    .iter()
-                    .map(|c| Category::from(c.as_str()))
-                    .collect::<Vec<_>>(),
-            )
+            .categories(categories)
             .build();
         Ok(item)
     }

tests/cmd/rss.out/_dest/rss.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?><rss version="2.0"><channel><title>My blog!</title><link>http://example.com</link><description>Blog description</description><item><title>My fifth Blogpost!</title><link>http://example.com/posts/my-fifth-blogpost.html</link><description><![CDATA[<h1>My fifth Blogpost!</h1>
 ]]></description><guid>http://example.com/posts/my-fifth-blogpost.html</guid><pubDate>Tue, 16 Feb 2016 10:00:00 +0100</pubDate></item><item><title>My first Blogpost</title><link>http://example.com/posts/my-first-blogpost.html</link><description><![CDATA[It's my first blog post]]></description><category>blog</category><category>first</category><guid>http://example.com/posts/my-first-blogpost.html</guid><pubDate>Fri, 01 Jan 2016 21:00:00 +0100</pubDate></item><item><title>My fourth Blogpost</title><link>http://example.com/posts/my-fourth-blogpost.html</link><description><![CDATA[<h1>My fourth Blogpost</h1>
 ]]></description><guid>http://example.com/posts/my-fourth-blogpost.html</guid><pubDate>Fri, 29 May 2015 23:00:00 +0100</pubDate></item><item><title>My third Blogpost</title><link>http://example.com/posts/my-third-blogpost.html</link><description><![CDATA[<h1>My third Blogpost</h1>
-]]></description><category>test</category><category>blogpost</category><guid>http://example.com/posts/my-third-blogpost.html</guid><pubDate>Wed, 27 May 2015 23:00:00 +0100</pubDate></item><item><title>My second Blogpost</title><link>http://example.com/posts/my-second-blogpost.html</link><description><![CDATA[<h1>My second Blogpost</h1>
-]]></description><guid>http://example.com/posts/my-second-blogpost.html</guid><pubDate>Fri, 02 Jan 2015 10:00:00 +0100</pubDate></item></channel></rss>
+]]></description><category>test</category><category>blogpost</category><category>blog/test</category><guid>http://example.com/posts/my-third-blogpost.html</guid><pubDate>Wed, 27 May 2015 23:00:00 +0100</pubDate></item><item><title>My second Blogpost</title><link>http://example.com/posts/my-second-blogpost.html</link><description><![CDATA[<h1>My second Blogpost</h1>
+]]></description><category>lorem/ipsum</category><guid>http://example.com/posts/my-second-blogpost.html</guid><pubDate>Fri, 02 Jan 2015 10:00:00 +0100</pubDate></item></channel></rss>