slt-release-finish,start: gitflow release tools

These tools implement the gitflow release process, auto-generating the
changelog and setting the npm package version.

/to @rmg @bajtos 

I needed to cut a strong-supervisor release that was not from the head of origin/master, unsupported by hubflow, so I finally replaced it with a few lines of shell scripts, as threatened.
This creates a tag `1.5.0` with a commit message saying `v1.5.0`.

We were doing it the other way around in loopback - the tag is called `v1.5.0` and the commit message is `1.5.0`.

I don't care much about the commit message, however the tag name should have the prefix `v`