Add a shareProvider extension for Pinboard.in

Hello! In addition to RSSOwl, I use the bookmarking site [Pinboard](https://pinboard.in/tour/). I would like to be able to add articles from RSSOwl to my Pinboard. This patch is intended to add Pinboard to RSSOwl's list of "Sharing" links.

Note that since I am new to Java, I have not yet built and tested this addition. I have modeled it exactly after the other `shareProvider` extensions. The Pinboard API requires HTTPS, and I notice that no other extension URLs use HTTPS, so I am unsure whether this is something RSSOwl can handle.