Display usage when running without argument (or with `-h`)

[tenzap]

When running css-validator from the command line without any argument, or with -h nothing happens. The output is confusing. It works with --help, but actually that's by accident because this argument is not supported.

Could you please fix this? By either supporting -h, or displaying the usage when no argument is given?

$ java -jar css-validator.jar -h
{output=text, profile=css3svg, vextwarning=true, warning=2, medium=all, lang=en}

$ java -jar css-validator.jar
{output=text, profile=css3svg, vextwarning=true, warning=2, medium=all, lang=en}

$ java -jar css-validator.jar -help
{output=text, profile=css3svg, vextwarning=true, warning=2, medium=all, lang=en}