Add markdown format support (#25)

Co-authored-by: Dawid Dziurla <dawidd0811@gmail.com>

node_modules/showdown/test/features/metadata/simple-three-dashes.md

@@ -0,0 +1,16 @@
+---
+title: This is the document title
+language: en
+author: Tivie
+contributors: John, Mary, Steve
+description: This is a long text and so it
+    spans on multiple lines.
+    It must be indented,
+    for showdown to parse it correctly.
+    Markdown **such as bold** is not parsed
+    and it will be rendered as plain text.
+date: 01-01-2010
+keywords: foo, bar, baz
+---
+
+**some** markdown text