Update completion for heroku

* Update descriptions
* Add `config:get`
* Rename `config:add` -> `config:set`
* Rename `config:remove` -> `config:unset`

These changes are described at:
https://devcenter.heroku.com/articles/config-vars#setting-up-config-vars-for-a-deployed-application
Merge pull request #239 from tricknotes/update-heroku

Update completion for heroku