add a new option to prefix 'Generated by PureScript version X.Y.Z' comment

issue #557.
A new option -g --generated-by has been added. if specified, it will add //Generated by PureScript version X.Y.Z to each js, where X.Y.Z is reading from Cabal. 

Doesn't this give a warning for no type signature with `-Wall` turned on? In any case, can we add one please? Thanks.