ipfs config cmd with git-style get/set

Adds "ipfs config" command designed in git style.
See specific config's values with:

        $ipfs config datastore.path
        ~/.go-ipfs/datastore

Assign a new value with:

        $ipfs config datastore.path ~/.go-ipfs/datastore

Open the config file in your default $EDITOR:

        ipfs config edit
config: cleaned up ipfs config cmd
config: redo to use other serialization functions
linted config
Merge branch 'master' of https://github.com/jbenet/go-ipfs