Add markdown format support (#25)

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

action.yml

@@ -33,6 +33,9 @@ inputs:
     description: Content-Type HTTP header (text/html or text/plain)
     required: false
     default: text/plain
+  convert_markdown:
+    description: Convert body from Markdown to HTML (set content_type input as text/html too)
+    required: false
   attachments:
     description: Files that will be added to mail message attachments (separated with comma)
     required: false