Feature, implement :main command

[Neil Mitchell]

> Haskell 98 libraries?  How quaint.
Up until I started working on WinHugs, I was using a 2001 release of
WinHugs, which didn't really work with much beyond Haskell 98
libraries...

> This differs from setArgs in that it leave the args set.  Maybe the
> command should just set the args.

>From what I can tell, setArgs leaves the arguments set, but withArgs
doesn't because with changes the arguments back after the computation
has run. My initial thought was to revert the arguments back
afterwards, but it wasn't immediately clear how to do this - because
of things like longjmp.