Better `--open` functionality on Linux

[NfNitLoop]

See: https://github.com/NfNitLoop/feoblog/blob/ccb906cab1cb350833c911eb61e28aef9027b89a/src/server.rs#L84

Right now I'm just using the webbrowser crate to open a browser, but on Linux that might open a blocking browser, which will block the server from actually starting.

There's an open issue for that here: https://github.com/amodm/webbrowser-rs/issues/18

--open still might work for a user, depending on what they have set up as their default browser, and which launcher it can find, so it's not necessarily fully broken, but it would be nice if this were consistent with Windows & MacOS behavior.