Check if the version has been already released

At the moment it is possible to release the same version twice which I guess is not very good.

I added version check which will halt the release if the same version is already there.

Version rewriting (although it obviously contradicts the “versions are immutable” part) should (1) require some extra flag (e.g. `--force` as in `git push --force`) and (2) rewrite the existing version, not make a new one.

The changes are not so huge as it seems. I had to move JSON reading and parsing earlier to avoid creating files and stuff.