Report feature availability via corelist -f

Report feature availability via corelist -f

> regen/feature.pl contains the data below, which could be used to add
> information to Module::CoreList to show what feature was available
> since what perl version
>
> $ corelist -f switch
> Data for 2013-01-20
> feature switch was first released with perl v5.9.5
>
> Any takers?

I reached for this the other day and was sad it didn't already exist.
Patch attached.

As suggested by H.Merijn Brand.

Signed-off-by: Chris 'BinGOs' Williams <chris@bingosnet.co.uk>