Removed config.Filename and replace it with config.GetConfigFilePath that
takes a configuration directory as argument instead. Makes code simpler.
ipfs.getConfigDir now also return the default configuration dir instead of
an empty string in some cases.
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