Configurable path for environment "dot" files

As per laravel-dev discussion, adds the ability to specify a path other than path.base that contains environment "dot" files.

The particular use case is a server with multiple laravel codebases that share the same configuration information (S3 keys etc), should be able to have their *.env.php external to the project root for ease of configuration.

I have created an associated pull request for laravel/laravel that adds a path.env entry to paths.php