Adding support for outputting Lint XML

We wanted to use scss-lint in our Continuous Integration environment so we needed the output in a format parsable by Jenkins.
This patch adds a ```-x``` option to scss-lint in order to output Lint XML as csslint does. In this way we can use scss-lint on Jenkins, setting its output format as csslint.


I'm not a Ruby developer so the code is probably dirty and could be improved. Sorry for that :)