ipfs config command with git-style option value getters and setters

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(as taken from $EDITOR enviroment variable):
        ipfs config edit