export source links to yaml header in markdown output

[dsteinkopf]

I think it is a good idea to have to original link url in the exported markdown ouput.
Example:

---
author: me
comments: true
date: 2015-12-19 11:21:23+00:00
layout: post
link: https://my-url.gobal.link/2015/12/foo-bar/
...

---

foobar

I'd be happy to see this merged. Thank you.
BTW. Thank you for exitwp!

[thomasf]

Yeah it seems to make sense to do this by default. thanks