(Also) Support `task <filter> [<reportname>] export`

This is a sequel to #2576 , which did introduce in a "nice improvement", by supporting `task export []`.

However, the syntax feels, to me at least, a bit "un-intuitive".

`task [] export` would probably feel more intuitive to many unix users, i.e sort-of "piping" the output of `task []` to `export`.