print usage: print options before arguments, add optional prefix

[thomas-mc-work]

• it's common to print the cli arguments after the options (on some OS it's even mandatory to adhere to this rule)
• I added an optional parameter to the printSingleLineUsage method to add a custom prefix to the output. This is a convenient shortcut to precede with the invocation of the program itself, e.g. java -jar my-app.jar ...

[thomas-mc-work]

ping @kohsuke