slt-changelog: --version & --sumary work together

Using "--summary --version X.Y.Z" now generates output suitable for
piping to `git commit -F-` for the version change commit.

Also cleanup old copy/paste code for supporting working outside of a
git repo.
Add usage to 'slt version'
slt-release-finish,start: gitflow release tools

These tools implement the gitflow release process, auto-generating the
changelog and setting the npm package version.
fixup! include this versions log in CHANGES
fixups

- more explicit pulls
- use -D to delete release branch
- checkout production to make it easier to do npm publish
- correctly generate the changelog
- add a slt-release one-shot command to start/finish
fixup! more explicit push command
Push results to origin, publish to npmjs.org