fix(contrib): Fix markdown output of changelog script
fix(contrib): Fix markdown output of changelog script

Now the script will generate markdown lists just as the current
changelog in deis/deis:CHANGELOG.md is in.

Thanks to @bacongobbler for the insight about `--format` over
`--pretty=format` and unneeded newlines not being echoed.

deis/workflow,deis/workflow,deis/workflow