Sign Windows application with external tool

in #109 @paulcbetts wrote: "It signs all of your executables and update.exe, then signs the outer MSI/EXE"

Is it possible to sign the executables with a different tool? We have an enterprise application signing tool that keeps certificates off of our development machines.

I am writing code to

1. run electron-packager on our project
2. sign the output of electron-packager
3. call electron-wininstaller to create the installer
4. sign the output of electron-wininstaller

Unfortunately, update.exe is not available in any of these steps.

A separate issue that the `resources/*.asar` files are not signed. This leaves a potential attack vector open. https://github.com/electron-userland/electron-packager/issues/656