Added support for xml output in scan command

Implemented the xml option in `ScanCommand`.

I also changed target to an argument instead of an option (since it is required) and simplified some of the option handling by adding defaults and specifying value-required where values are expected.

O, and I also switched from `echo` statements to `$output->write()`. This seems to me to be more in line with the indended usage of symfony console.

As far as I can see these changes does not affect the functionality of parse in any way. Hope you like it.

/Hannes