Automate the Hugo release process

This commit adds a work flow aroung GoReleaser to get the Hugo release process automated and more uniform:

* It can be run fully automated or in two steps to allow for manual edits of the relase notes.
* It supports both patch and full releases.
* It fetches author, issue, repo info. etc. for the release notes from GitHub.
* The file names produced are mainly the same as before, but we no use tar.gz as archive for all Unix versions.
* There isn't a fully automated CI setup in place yet, but the release tag is marked in the commit message with "[ci deploy]"

Fixes #3358

docs/content/release-notes/_index.md

@@ -0,0 +1,8 @@
+---
+date: 2017-04-17
+aliases:
+- /doc/release-notes/
+- /meta/release-notes/
+title: Release Notes
+weight: 10
+---

docs/content/release-notes/release-notes.md

@@ -2,12 +2,8 @@
 aliases:
 - /doc/release-notes/
 - /meta/release-notes/
-date: 2013-07-01
-menu:
-  main:
-    parent: about
-title: Release Notes
-weight: 10
+date: 2017-04-16
+title: Older Release Notes
 ---
 # **0.20.2** April 16th 2017