Use default target if none specified

[dlindenkreuz]

Currently, the component does not do anything if no target prop was specified.

It probably should use window or at least warn about a missing target.

[oliviertassinari]

I think that we should make the target required. This package used to be a mixin, we hadn't the option at that time.

Providing a default value sounds like a bad idea, some events can only be listened on the document and window respectively. The choice needs to be explicit.