Enable autostart of service after install

Not sure if this is a feature anyone else wants. I bumped the nssm binaries to the latest pre-release of 2.21 to get the start command for nssm supported. I then added a -a argument that will cause the service to autostart. It's something we needed in our usage, so I thought I'd send a PR in case it's useful/acceptable.

No worries if not, we're just running off this fork for now.

Thanks for this lib!