Support mailto: in links

Fixes https://github.com/kaushalmodi/ox-hugo/issues/149.

ox-hugo.el

@@ -1822,7 +1822,7 @@ and rewrite link paths to make blogging more seamless."
      (t
       (let* ((link-param-str "")
              (path (cond
-                    ((member type '("http" "https" "ftp"))
+                    ((member type '("http" "https" "ftp" "mailto"))
                      ;; Taken from ox-html.el -- Extract attributes
                      ;; from parent's paragraph.  HACK: Only do this
                      ;; for the first link in parent (inner image link

test/site/content-org/all-posts.org

@@ -2455,6 +2455,10 @@ Once that is evaluated, links like these will export fine i.e. no
 #+begin_src org
 [[gopher://some.gopher.site][Dummy gopher link]]
 #+end_src
+*** Mailto link support
+{{{oxhugoissue(149)}}}
+
+[[mailto:abc@xyz.com][My email]]
 ** Within the same post (Internal links)                     :internal_links:
 *** Link to headings by name                     :toc:headings:export_option:
 :PROPERTIES:

test/site/content/posts/external-links.md

@@ -37,3 +37,10 @@ Once that is evaluated, links like these will export fine i.e. no
 ```org
 [[gopher://some.gopher.site][Dummy gopher link]]
 ```
+
+
+## Mailto link support {#mailto-link-support}
+
+`ox-hugo` Issue #[149](https://github.com/kaushalmodi/ox-hugo/issues/149)
+
+[My email](mailto:abc@xyz.com)