Add `@showln`, to display expression values and also indicate position in code

I've found this macro useful in my own work, particularly for tracking the value of a variable that changes during execution of a function.

```
function testme()
    x = 7
    @showln x
    x += 2
    @showln x
    x
end

julia> testme()
x => 7
  (in testme at /tmp/testme.jl, line 3)
x => 9
  (in testme at /tmp/testme.jl, line 5)
9
```

It would mean adding to exports, which means it needs to cross a "sufficiently useful" bar. I'm unsure myself, particularly since I'm hoping an integrated debugger will someday make this unnecessary. But I thought I'd share it and see what people think.