Add webmention client code to Junco

Access these tags and links:

* #webmention
* #indieweb
* [[IndieWebCamp principles for building the indie Web]]
* http://webmention.org
* http://indiewebcamp.com/webmention
* https://github.com/phpish/webmention 


"The ability for one webpage to let another know the first has linked to the second is a vital indieweb building block.
The key mention technology for the indieweb is: webmention"

#todo
#junco