Conditionalize onExit / signal handling

[movitto]

Greetings! Thanks for this great library, we've incorporated it into a fork of the 'jsonpath' library where we swapped the unsafe static-eval backend w/ v8-sandbox to safely process arbitrary user-specified expression.

In the application which uses jsonpath-sandbox we capture process-terminating signals and gracefully shutdown operations, restoring data to a consistent state before terminating. Part of this invokes the v8-sandbox shutdown method to cleanup operations here. The problem is that v8-sandbox utilizes the onExit function in the signal-exit module to catch error / exceptions on its own and invokes shutdown locally. Thus if the client process receives a signal while processing expressions that operation will not have the chance to gracefully terminate, handle the results and data could enter an inconsistent state.

Would it be possible to add a Sandbox constructor option to skip over the 'onExit' call and disable internal signal handling? If this is acceptable and it's just a matter of implementing it I'd be happy to send a PR.

Thank you for your consideration