update the artifacts created by earlier version of specify

[yubrshen]

Here is the use case:
update the artifacts created by earlier version of specify and potentially updated by the me (programmer), when specify has been upgraded.
Here is the manual process, for example:

1. Run specify init a-new-sandbox-project
2. Merge the artifacts in a-new-sandbox-project to those at my existing project initialized by the earlier specify and updated by me.

I wish there is a specify sub-command to automate the above process.

If it's already implemented please let me know. Or please share better ideas. Or I'll create an issue to capture the requirements.

This is the same as #728 , I believe, to have a clearer title, I posted separately here.