Inconsistent quoting in output of `config -s`

[edmorley]

heroku config -s currently gives output like:

HEROKU_APP_NAME=foo-bar
HEROKU_RELEASE_CREATED_AT='2016-09-29T11:05:02Z'
HEROKU_RELEASE_VERSION=v58
NEW_RELIC_CONFIG_FILE='newrelic.ini'
WEB_CONCURRENCY=3

ie: some values are quoted with ', others not.

Whilst the unquoted entries don't technically need quotes, I think it would be more consistent to just always include the quotes around each value.

Currently the npm package shell-escape is used: https://github.com/heroku/heroku-apps/blob/c402fef1efc02a350f370eab48be06dcd8475663/commands/config/index.js#L14

...which doesn't have a way to force quotes: https://github.com/xxorax/node-shell-escape/blob/master/shell-escape.js