feat(update): add pre-stable warning message with docs link

[acburdine]

closes #364

• if an instance was installed with a pre-stable version of Ghost-CLI, this message is output so that people know they need to do things to update their instance to work with the newest CLI version

[acburdine]

cc @ErisDS for a wording check. I went ahead and added a docs section at the link outlined in the PR - but in the case that the link changes I wanted to hold off on merging this.

Feel free to merge though if the link looks good / the docs are correct 👍

[coveralls]

Coverage remained the same at 57.848% when pulling d9541f9 on acburdine:warn-pre-1.0 into bc845de on TryGhost:master.

[ErisDS]

@acburdine Link is now https://docs.ghost.org/docs/how-to-upgrade-ghost#section-upgrading-ghost-cli

Slightly different wording:

Ghost was installed with Ghost-CLI v${instance.cliConfig.get('cli-version')}, which is a pre-release version. Your Ghost install is using out-of-date configuration & requires manual changes. \n

[acburdine]

@ErisDS done - good to review again

[coveralls]

Coverage remained the same at 57.848% when pulling 4225a55 on acburdine:warn-pre-1.0 into bc845de on TryGhost:master.