Add optional release version suffix

[M3t0r]

The purpose of this pull request

To make it easier for users of gollum to build their custom versions that can also include the versions of their own plugins without manually adding to the gollum version as defined in git and the Makefile.

We are already using this, I just forgot to make a PR last year. Sorry 😞

Config to verify

make all GOLLUM_RELEASE_SUFFIX=1.2.3
make all

Checklist

• [x] make test executed successfully
• [ ] unit test provided
• [ ] integration test provided
• [x] docs updated

[M3t0r]

The GitHub UI confused me. This already was merged. But it still shows me the diff in the pull request, and tells me my branch wasn't merged yet.

Never mind.