run emrun on localhost / 127.0.0.1 by default

Currently `emrun filename.html` listens to `0.0.0.0`. Instead it should listen to `localhost` / `127.0.0.1` by default.
_(checked version `3.1.46`)_

For security reasons debugging and testing tools like emrun should not listen to possibly publicly reachable addresses.
Yes, there might be a firewall protecting public addresses. But it's better to pick safe defaults.
_(Yes, even the localhost address isn't extremly safe. But that [will](https://wicg.github.io/private-network-access/) [hopefully](https://developer.chrome.com/blog/private-network-access-update?hl=en) [change](https://bugzilla.mozilla.org/show_bug.cgi?id=354493) [soon](https://bugzilla.mozilla.org/show_bug.cgi?id=1481298))_

In case it's needed `emrun` already has a `--hostname` switch where you can manually set `0.0.0.0` to listen on all addresses.