Dvc: config: add support for --show-origin

[efiop]

Same as git config:

$ git config --list --show-origin
file:/home/efiop/.gitconfig     [email protected]
file:/home/efiop/.gitconfig     user.name=Ruslan Kuprieiev
file:.git/config        core.repositoryformatversion=0
file:.git/config        core.filemode=true
file:.git/config        core.bare=false
file:.git/config        core.logallrefupdates=true
file:.git/config        [email protected]:efiop/dvc
file:.git/config        remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
file:.git/config        branch.master.remote=origin
file:.git/config        branch.master.merge=refs/heads/master
file:.git/config        [email protected]:iterative/dvc
file:.git/config        remote.upstream.fetch=+refs/heads/*:refs/remotes/upstream/*
file:.git/config        branch.fix-4050.remote=origin
file:.git/config        branch.fix-4050.merge=refs/heads/fix-4050
file:.git/config        branch.fix-4162.remote=origin
file:.git/config        branch.fix-4162.merge=refs/heads/fix-4162

i'm not personally sold on file: prefix in path (just because I don't really know why git does that, maybe there is a good reason), but the rest looks pretty good. We now have dvc config --list, so now we need dvc config --list --show-origin as the next step.

This will be realy handy to have for users to figure out where the configs are and also handy for us, developers, when debugging particular issues.

[mrstegeman]

I'll pick this up and submit a PR soon.