[Git submodules] Bump to the latest release

Hi,

First of all, thank you for this great tool and for providing it freely for open-source projects :heart:

I'm opening this feature request because the Git submodules updater always bumps to the latest commit of a specific branch. It works perfectly well but, not all projects use a dedicated branch for stable code. This is why I would like to be able to bump to the latest release (git tag) present in a repository. This [API call](https://developer.github.com/v3/repos/releases/#get-the-latest-release) can be used for this purpose.