ipfs config command: set and get configuration options

ipfs config  command allows to set and get IPFS configuration option values.

To open the configuration file in $EDITOR run:
  ipfs config edit

One can see specific config values:
        $ ipfs config datastore.path
        "~/.go-ipfs/datastore"

Setter:
        $ ipfs config datastore.path "~/.go-ipfs/datastore"
is not implemented here yet.