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. 

This needs a space to be matched by GitHub's regex:

```
// Generated by
```

Also maybe "Generated by _psc_" would be better in case we ever want to distinguish other compilers later (purescript-in-purescript maybe?)