Allow an option to check that plugins have changed since .cache

[mykeels]

I'm receiving the following info during gatsby build:

One or more of your plugins have changed since the last time you ran Gatsby. 
As a precaution, we're deleting your site's cache to ensure there's no stale data.

A CLI option that checks whether the plugins hash is the same with the one saved in the cache would be great, so I can redownload the .cache and public folders from S3.