Add SECTION_CAPTION paragraph type

This doesn't seem to be rendered on medium.com. Here's a post that has
one, but the text is nowhere on the page:
https://medium.com/message/the-joy-of-typing-fd8d091ab8ef

This help articles hints that it might have been a feature at one point
that they don't allow anymore:
https://medium.com/@Medium/images-652ee60abea6

src/classes/paragraph_converter.cr

@@ -47,6 +47,11 @@ class ParagraphConverter
         paragraph = paragraphs.shift
         children = MarkupConverter.convert(paragraph.text, paragraph.markups)
         node = Preformatted.new(children: children)
+      when PostResponse::ParagraphType::SECTION_CAPTION
+        # unused. just here to catch the type instead of implicitly going to the
+        # else block
+        paragraph = paragraphs.shift
+        node = Empty.new
       when PostResponse::ParagraphType::ULI
         list_items = convert_uli(paragraphs)
         node = UnorderedList.new(children: list_items)

src/models/post_response.cr

@@ -52,6 +52,7 @@ class PostResponse
     P
     PQ
     PRE
+    SECTION_CAPTION
     ULI
   end