Add scripts to help maintain version tags and changelog entries

scripts/tag-version.sh will tag the current commit with the version number found in the VERSION file. You should have already updated the VERSION file and committed the changes.

scripts/gen-changelog.sh will prepend the CHANGELOG file with a summary of the pull request commit message bodies, up to the last tagged commit. You should edit the resulting CHANGELOG file before committing it, and you should have already bumped the VERSION file to the next version. This change can be committed in the same commit as the VERSION bump.