Script for bumping the chart version

Closes: https://gitlab.com/charts/gitlab/issues/481

Notes:

• This script is not branch aware, stable branch detection/creation will be up to the release script
• Pull in the version.rb from release tools, adds a diff method
• Changelog get's it own version class, but remains otherwise the same for now. Now that we are using semver versions, our changelog can change to be more inline with release-tools, but we will handle that later
• Tests are added, but are not run by CI at the moment.