This rewrites the release logic to use CircleCI 2.0 and its approve workflow in combination with the state of the release notes to determine what to do next.
Fixes#3779
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
Put version handling into the helpers package so it can be used by many,
and split version and suffix to make it possible to calculate the next Hugo version.
