add RSS Blue support for Medium

Supporting `podcast`, `music`, `audiobook`, `newsletter`, and `blog`
values.
Test feed: <https://rssblue.com/@test-podcast-3/feed>.

docs/element-support.md

@@ -102,3 +102,6 @@ For elements that are included in the official [DTD](https://github.com/Podcasti
 1. [Castos](https://castos.com/earn-bitcoin-from-your-listeners/)
 2. [usocial](http://usocial.me/history#v0.1.1)
 3. [RSS Blue](https://rssblue.com/help/podcast-metadata#lightning-node)
+
+## Medium `<podcast:medium>`
+1. [RSS Blue](https://rssblue.com/help/podcast-metadata#medium)