Allow custom handler for clicks outside modal

[lemonad]

When clicking outside the modal, it is automatically closed, which is probably what most people want. In my particular use case though, I don't want users accidentally closing the modal since it is showing the progress of a software update.

It would be nice if it was configurable, preferably by being able to pass a custom handler for clicking outside the modal.

[xstevenyung]

Totally agree with this. Would be totally open for a PR 👍