The cache strategy of merged config files

[roxblnfk]

I propose new default dehavior: merge in runtime all configs that are marked as optional in the composer.json (with ?). ENV params will be determined there too.

And everything else will be always cached and updated on composer update/install:

• files from vendor packages
• project files that has not been marked local or optional

Most projects will benefit from this: it will be convenient for the developer to change local configs and not call the composer du command manually.
For more loaded projects, provide option for full caching of configs (as it happens now) and rebuilding on the comoser du command.