Create a Namespace helper for initializing a package

[wchaudry]

When initializing with wapm init the name of the package suggested should concatenate the username + directory, otherwise, when publishing, it complains about “Error: Global use for aliases is disabled for now. Try renaming your package to: 'francisco-perez-sorrosal/simple_wasmer' or use a different namespace”

Action: Provide the default user namespace Alternative: Provide a list of namespaces the user might be a member of.