Local config is needed

[dmpetrov]

Some of the config settings are not general enough to submit into a repository but still important to keep in a local setting environment.

Examples:

1. Cloud storages can be set up locally, not globally.
2. A stage that user would like to lock. DVC files and config is probably not the best place for that.

Possible solution: store a local config .dvc/config.local which can overwrite (global) config settings and is ignored by Git.

[efiop]

After a bit more thinking and tinkering, I think that we should not hurry with it and leave it for consideration after the 0.9.0 release. If we do decide to implement this later, there will be no problem with backward compatibility. Removing from the current project for now.

[efiop]

This is going to be essential for #596 . Already working on it.