Add a verbose argument to as_forecast()

[nikosbosse]

Just we've done with validate_forecast() in #778 we should add a verbose argument for as_forecast()

[seabbs]

I don't think its really needed right now. I would vote for closing this and seeing if there is user demand (either from us or from others)

[nikosbosse]

I think my personal user-demand would be to use this to get rid of a lot of print statements and suppressMessage() calls in the tests...
But yeah, definitely don't need to do that for the next CRAN release