[hump.signal] stopping signal propagation

Would be great to have a way to stop the signal propagation:
```lua
function Registry:emit(s, ...)
  for f in pairs(self[s]) do
    if f(...) == false then return end
  end
end
```

so we could stop the signal using:
```lua
Signal.register('mousepressed', function(x, y)
  ...
  return false
end)
```