Add 'message:' link type to default link types

* org.el (org-link-types): Add 'message:' link type to default link
types.

Jules Bean wrote:
>The link-type "message" is one of the ones org handles by default, it
>is an explicit case in org-open-at-point, much like http and it is
>handled by the following code:

>((member type '("message"))
>	   (browse-url (concat type ":" path)))

>However it is not included in the default value of org-link-types:

>(defvar org-link-types '("http" "https" "ftp" "mailto" "file" "news"
>		       		   "shell" "elisp" "doi"))

>...and therefore it doesn't work when clicked/followed.

>Manually adding it to org-link-types makes it work correctly. For me,
>anyway.

lisp/org.el

@@ -4755,7 +4755,7 @@ The following commands are available:
 
 (defconst org-non-link-chars "]\t\n\r<>")
 (defvar org-link-types '("http" "https" "ftp" "mailto" "file" "news"
-			   "shell" "elisp" "doi"))
+			   "shell" "elisp" "doi" "message"))
 (defvar org-link-types-re nil
    "Matches a link that has a url-like prefix like \"http:\"")
 (defvar org-link-re-with-space nil