add the option to add default namespace

[fpriyatna]

Currently, I have to use this command sed 's/<no value>//g' mapeathor_output.ttl

[anaigmo]

@fpriyatna can you provide an example of a mapping with the feature you request?

[dachafra]

If @fpriyatna won't provide the requested example in a couple of days we will close the issue