Hi!

I would like to submit a patch to org mode, which fixes the behavior
of mailto and irc links in markdown export.

Previously, these org mode snippets:

[[mailto:some.email@server.net][Jane Doe]]
[[irc:/irc.com/#emacs/bob][bob]]

would export to:

 [Jane Doe](some.email@server.net)
 [bob](/irc.com/#emacs/bob)

which causes broken links in browsers when they try to view exported markdown.

The new behavior exports them to:

[Jane Doe](mailto:some.email@server.net)
[bob](irc:/irc.com/#emacs/bob)

so that browsers can open the proper respective application when
opening said links.

The patch that I have made is very small (only one line), and is attached.

There is an external report of this bug here:
https://github.com/larstvei/ox-gfm/issues/17

This is my first time submitting a patch to anything big like org
mode, so please let me know if I've done anything wrong or if anything
seems amiss.

Thanks for looking at this, and thanks for making/developing on org
mode (it's changed my life completely)!

-Jay Kamat