Store vendor configs hash, use it to determine if file was changed

[samdark]

In order to improve experience we can store config hash in a file similar to composer.lock and then use it to determine if there were changes in vendor config. The goal is not to display any warnings if config is a custom one and vendor did not change since last time.

[samdark]

Alternatively it could be stored as a key in merge plan.

[samdark]

See #44 (comment). Likely we can avoid it.