commandline option to exit restview if webbrowser doesn't poll anymore

AFAICT the webpage that `restview` generates polls back for changes using JavaScript. It would be
useful to have a commandline-option/config-option/environmentvariable where you could specify
an amount X, so that if there was no polling for X seconds restview would exit. That way closing the
browser window is enough to stop the `restview` process.

I could take a shot at a PR if this is deemed useful.

Start by tracing the restview command-line entry point and the generated webpage's JavaScript polling behavior. Done means an option, configuration setting, or environment variable can specify an inactivity interval and restview exits after that interval without polling.

Written by the indexing model from the issue text.