Enable creating signed installers automatically

Having a signed installer is a critical part of making sure SmartScreen and virus scanners don't give you shit. This PR adds a new option to `--releasify` to allow you to do this, `-n`, where you pass the parameters you want to give to `signtool sign`. Here's a typical usage:

```sh
Squirrel --releasify myNuGet.nupkg -n "/a /f ./Certificates.pfx /p myCertificateSecretPassword"
```

It's important that Squirrel do this because the inner `Update.exe` needs to be signed, which would be super annoying to do outside of the Squirrel release process.