plum start runs in foreground by default

Also fixed LogPrinter regressions. Sorry for not doing that in a
separate commit.

Also made 'plum logs' show backlog. Yep, rolled that right in too. Gonna
go whip myself now.
Fix run: use Container.logs(), explicitly start container
Set port_bindings to None when starting a one-off container in 'plum run'
Hide stack traces for Docker API errors
Fix method name in start()