Add support for [YANKED] releases

[mrombout]

Yanked releases are versions that had to be pulled because of a serious bug or security issue. Often these versions don't even appear in change logs. They should. This is how you should display them:

[0.0.5] - 2014-12-13 [YANKED]

The [YANKED] tag is loud for a reason. It's important for people to notice it. Since it's surrounded by brackets it's also easier to parse programmatically.

• Parser should recognize the [YANKED] tag and set the Yanked field on that Release to true.
• gochange yank <release> should mark that release as [YANKED], or fail if that release doesn't exist.