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. 

It doesn't make sense to add the current prefix to the externs files, since the comment format for JavaScript is not valid PureScript syntax.