Expected Behavior / Situation

It seems appropriate for rollup -w to add a SIGINT handler in which watcher.close() is called, triggering closeWatcher hooks, so that plugins that have need of closeWatcher can properly clean up.

Actual Behavior / Situation

The closeWatcher hook is not called when the process from rollup -w is killed with SIGINT (e.g. via Ctrl + C).

Modification Proposal

Simply add a process.on('SIGINT', handler) in the cli handler for rollup -w to gracefully close the watcher so that closeWatcher hooks are called.

I can likely prepare this PR if desirable.