Adding command line in xunit plugin to specify test suite name

Hi guys, 

I opened a bug #859, as right now there is no way to specify test suite in xunit plugin as .  I have added one more command line paramter (--test-suite-name) through which you can specify the test suite name as well. 

adding this pull request to resolve #859
Since this is specific to the xunit plugin, it should probably be prefixed with xunit, like `--xunit-file`.