Make option syntax consistent with Dgraph Zero/Dgraph Server

Moved from GitHub ratel/30

Posted by danielmai:

This feedback comes from What's the correct syntax for parameters? - #2 by dmai.

Dgraph Zero and Dgraph Server uses Cobra which provides conventional double-hyphen options (e.g., --addr vs -addr and a command summary.

Would be nice to replace pkg/flag with Cobra for consistency.