support multipart email (#48)

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

action.yml

@@ -20,15 +20,18 @@ inputs:
   subject:
     description: Subject of mail message
     required: true
-  body:
-    description: Body of mail message (might be a filename prefixed with file:// to read from)
-    required: true
   to:
     description: Recipients mail addresses (separated with comma)
     required: true
   from:
     description: Full name of mail sender (might be with an email address specified in <>)
     required: true
+  body:
+    description: Body of mail message (might be a filename prefixed with file:// to read from)
+    required: false
+  html_body:
+    description: HTML body of mail message (might be a filename prefixed with file:// to read from)
+    required: false
   cc:
     description: Carbon copy recipients (separated with comma)
     required: false
@@ -41,10 +44,6 @@ inputs:
   ignore_cert:
     description: Allow unsigned/invalid certificates
     required: false
-  content_type:
-    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